There is a batch of older PC's(HP DC7200) with an extra graphics cards (nVidia Corporation G72 [GeForce 7300 SE/7200 GS]). In many of these machines the graphics card is going on the fritz, because the hardware is failing.
There is no more warranty on these cards.
To resolve the issue you can remove the card and use the "onboard" graphics adapter.
When you restart the PC you'll get a message telling you the graphics settings aren't correct any more.
Select the option to "reconfigure" the graphics and restore the "default", then select "cancel".
Now select "restart X". You'll get a different login screen, but it works like the normal screen.
After another reboot you'll get the normal login screen again.