|Usually means your profile is corrupt.|
Try starting in safe mode (keep tapping F8 key before windows starts loading) and log on to Administrator user (should have no password). If you get in here, create a new user account (with Administrator privileges), restart in normal mode and logon to new user. If this is ok, you'll need to move your documents etc from your old user profile (which you can find under documents and settings folder) to new. If you were using outlook or outlook express for email & need to rescue emails and/or address book, post back - its fairly straightforward.