HELP! Windows reboots now stuck in "Please Wait" cycle!

Hello,

I had just started up my computer today, checked email, done a few things online, and it was idling on the desktop while I took a call.
All of the sudden, it reboots.

A second later it comes back on. the BIOS splash screen shows up, then it proceeds to boot windows. The windows loading taskbar shows up, then the spinning Windows loading circle, and then the Windows "Welcome" screen is displayed.
A few seconds later, it fades to black and only the mouse pointer is visible. I can move the mouse all around, but the desktop remains black! Not a single thing is visible except the cursor.

I hard rebooted and went into safe mode, and the same thing!! Nothing except the cursor.

Finally, after tapping the power button, the Windows welcome screen appears with the words "Please Wait" and the Vista orb rotating indefinitely. I've let it sit there for upwards of 20 minutes and nothing seems to be happening. What on earth would do this?


Does anyone have any ideas?? I'm completely stumped! Is this a virus?? How can I check without getting to the desktop?
What on earth would just cause the desktop to go black, even in safe mode!?

Please help!!