Sounds like a program or driver that loads at startup is
causing this. Boot up in SAFE mode by tap-tap-tapping the
F8 key after the BIOS messages and before the Windows logo
comes up. Windows will look funny but if it comes up, you
have a clue. Use MSCONFIG to turn off auto-start programs
(Start, Run, type MSCONFIG, go to the Startup tab and
uncheck everything, hit Apply, then OK and restart). If
Windows appears to come back normally, use MSCONFIG again
to turn on ONE item at a time, re-booting after each.
Eventually you may locate the source of the problem. Good
Luck !