Dim L800CXE- Intermittent No screen visible

Dimension L800CXE. Machine was working fine for last 6-8 months. Brought machine to my house (long story). Boot-up: all normal till logon windows. Got outline of window and froze. Reboot to safe mode. froze during list of functions loading. Reboot- no video/screen. Monitor did not click on with green power light, like it should. Added vid card. Monitor came on. Wndows loading, with screen, and progress bar, almost to end, and stops. No more progress. Reboot- no monitor again. Remove vid card, plug back to built-in video, reboot, monitor comes on again. Suspect virus. Reboot to OS CD-ROM. Everything goes ok. Format drive, all OK. First auto restart- no monitor again. Now noticing power led and HD led not coming on (it was before). Possible power supply problem? Or M/B going south?