Could be the memory on your video card. Try a different card if possible.
To do a screen shot, touch control and print screen on your keyboard and open an image application..like Windows Paint if none other is available..click edit and click paste..that will put the image from the clipboard into Paint where you can "save as" (and give it a name). That will give you just the active screen that is open. You can just touch print screen and capture the whole screen and do the same thing. If you have Paint Shop Pro, you can use that..the same way..if you do not have anything other than Windows Paint, go download IrfanView
http://www.irfanview.com/ it is a freebie and does a great job..and you can save an image in other than BMP which is all Windows Paint will allow I think (in Win98 anyway).