as you've seen so far, there are a lot of possible causes for this sort of thing. ram is often the biggest culprit. ram, just like your harddrive, can get fragmented. just because you close an application doesnt mean it gets flushed from ram right away. applications like photoshop where you're often working with large files, draw a LOT of ram. games do this also. as ram gets stuffed with different stuff, sometimes it gets flushed and sometimes not.
there are basically two solutions. one, you get a memory flushing application and do it manually. and two, you reboot the machine. that will work for normal ram but i think only the normal ram gets flushed with manual flusing; not sure if the video ram does or not.
another solution is add more ram. i used to like to have at least 512 megs of ram in a windows machine. then i raised this to 1 gig. i'm now running 2 gigs. it does help. if you're running windows 98, there was a basic paging fault that caused a bottleneck to speed. you still had use of all your ram, but the access times didnt speed up page a certain point. xp, win2k and all the NT types dont have this bottleneck.
another solution is to upgrade to a pci-e type machine. the entire network of busses, kart and all that internal bit passing stuff was re-worked in pci-e and the benefits are quite obvious. it's like the difference in having one lane of traffic as opposed to two.
as someone else mentioned, trim your background programs. programs that stay resident in your system tray eat resources on a constant basis. and, more often than not, even if you 'exit' or 'close' them, they are still resident and running in the background. you need to stop them from loading at startup in many cases to keep them from eating resources.
trimming 'services' is another way to trim the fat from windows. there are a LOT of services that by default run in windows xp. a lot of these are not needed, especially on 'auto' mode. many do quite nicely set at 'manual' where they only start up when needed. and quite a few arent needed by anyone and can be turned off completely. see the blackviper archive listed in the library for help on that.
drivers. drivers are the main reason for horizontal and vertical patterns showing up on a monitor. this is usually video card drivers and not monitor drivers but dont neglect monitor drivers either. however, if you're only getting your streaks and patterns showing up once in a while, i'd say it's a ram/resource thing. if you're getting the streaks and patterns ONLY when using certain apps, then it's most likely drivers. games are actually a good way to find out if your vid drivers are the best for your system configuration.
assuming you're running windows xp, there are actually quite a few tweaks you can do to fine tune your system for better resource handling. right click on your desktop and pick 'settings' from the menu. hit the 'advanced' button and that shld take you to a popup window with various controls and settings for tweaking your display and how your video card interacts with the rest of your system. depending on if you have an ATI or nVidia card, you may have software that runs concurrent with this and adds extra tweaking features.
you can also right click on the 'my computer' icon and pick 'properties'. this will take you to another popup window with some additional broad tweaking items, one of which deals with priorities on your system such as giving priority to programs or background services. you'll also find a button for changing how much virtual memory you're using. you might be able to increase your virtual memory and handle some of your problems with that, but just remember that that is a poor substitute for real ram.
clean out your caches and temp files. the more windows has to check thru in caches and temp files to find things, the longer it takes to refresh.
defrag your drive(s).
do a registry clean up. the more stuff you add to your computer, the larger the registry gets. the larger the registry gets, the slower things refresh and boot up. this is similar to caches and temp files in this regard.
increase your video card ram. it means buying a new vid card, but sometimes it's worth it.
one of the hardest things for windows to deal with, for intel to deal with and for all the other software and hardware vendors to deal with, is ram and the flushing of same. when do you return ram for general use when it's been allocated to something. it's a trick. and, it's the biggest reason for the symtoms you're talking about. i have several programs that when ram gets short, they'll either not start correctly or crash soon after starting. they get slow and processes that would normally take seconds start to take minutes. ram gets fragmented. resources dont get returned and things start to bog down. i have one program, a game actually, that will quit loading various objects in the game. i know they shld be there, but they arent. there just isnt enough ram to load them. windows xp is far more robust than win98 ever was and it seems to have learned to turn things off rather than crash, like win98 did so frequently. but, it's still a problem.
you might also check your bios settings. agp cards, if that's what you have, had some odd things in bios. frankly, i dont recall them all, but i do recall turning some junk off in bios that helped. and i also remember something in there about page file memory allocations... i think.
and, since you didnt mention what type of card you have, just in case you have a pci vid card, there are specific settings for those as well, that are quite different from agp and pci-e cards. again, it's been a while, but i know there was one in desktop\settings\advanced that made a major difference in how the card was addressed and handled.
lol. the more i write, the more i remember.... you might also want to check what version of directx you're using and run the tests in dxdiag and even upgrade directx if need be.
also, and this can actually be a biggie... the latest and greatest drivers for your card may NOT actually be the best for YOUR system. i know that goes against the grain of everything you hear, but it's true. YOUR system was built at a time when directx was maybe a different version. the software was a different version, the game was made 5 years ago, and all this stuff was designed to work with drivers that existed back then, but maybe not now. so, sometimes you have to go retro on vid card drivers to find the ones right for your system.
ok, so i've written a book here. that's enough for now

just one last thing; get ahold of 3dmark or something like Everest. both are excellent complete analyses of your system type programs. Everest used to be free. it's not now but it's a good informational type program. windows is very lax on giving and organizing what every pc owner shld be able to see at a glance about their system.
ok, so i lied. there is more... get a heat monitor, the software kind that you install and it will show you some, most or all of your internal heat monitors. heat can actually screw up your software because it will screw with ram. and get a memory checker also. and, if you know how, do a watts power consumption analysis of your system. if you have a LOT of drives, cd roms, dvd players/burners and so on ad infinitum, you may be suffering from low power to some devices. this can affect your display also.
in short, however, i'd say more ram and check your drivers. frankly, i'm surprised i remember half of this stuff
