Video artifacts

In recent weeks, I have started experiencing video artifacts which cover and obscure information on the screen on my
monitor making my computer virtually unusable. I have tried everything that I can think of to correct the problem but
without success. My computer is a Dell Dimension 8300 and originally came with an nVidia GeForce 5200. Not long after
purchasing and receiving the computer, I replaced the GeForce 5200 with an nVidia GeForce 6800 Ultra and that is now the
current configuration. I have since tried reinstalling the oridingal 5200 but was again unsuccessful and the problem
persists. I have also now replaced the dynamic RAM with 4GB or new memory but nothing has changed. I have also tried
reinstalling the drivers for each. Even though Microsoft ceased supporting Windows XP, I have continued to use XP as my
primary OS. When MS announced that it would no long support XP, I purchased and installed Windows 7 but found that much
of my expensive software which was purchased to run on Win XP would neither install or run on Win 7. BTW, the video
artifact problem exists when running either Win 7 or Win XP. Any help with correcting this problem would be greatly
appreciated.