Did you make any changes just before this started? Add/remove hardware/software, make any updates, remove/quarantine a virus or spyware?

See if it happens booting in safe mode (F8 before windows screen at startup)
Sounds like something (program or driver) trying to load at the logon screen and failing.
Make sure you have the latest video driver for your system and unplug/disable any un-necessary hardware (external drive, bluetooth etc.)

If none of this helps you can try system restore to a point before this began.