...taking the cover off the computer, (Make sure to power down and unplug from the wall before doing this), then remove the video card, then reseat it again. If there are any bad connections, this should get them going again...
In addition, while you've got the cover off, blow out all the dust and junk from inside so it will stay cooler.
Hope this helps.
Here is all of the info that I can tell would be relevant:
NVIDIA GeForce Ti 4200 with AGP8X
Windows XP Home
Maxtor 80G hard drive - only 13G being used
AMD Athlon XP 2200 Processor
This computer is a couple years old and has never given me a problem, display wise, until now. At start-up while everything is checking out, the screen looks like the computer screens on The Matrix... jumbled symbols spaced sporadically all over the screen. After Windows opens there are several small, faint lines surrounding the cursor and the screen flashes off and on maybe two or three times a minute and sometimes goes black for a few seconds. It has frozen up a couple of times. I went to the NVIDIA website and updated the drivers. I have gone to Device Manager and disabled the display driver and the problem goes away. I don't use the computer for gaming or anything that would strees it's capabilities so I'm at a loss as to why this is happening. Any advice at all would be greatly appreciated! Thanks