Start with the obvious:
Press F8 while booting your computer, to see if you can get into Safe Mode. Assuming that you can, your screen will look very odd (and extremely basic) because the video driver won't have loaded, but you can still follow this route to run System Restore:
Start > Programs > Accessories > System Tools > System Restore.
Select a date prior to when the problem first arose, and restore your computer's registry to that date. (You won't lose any data, such as emails, documents and pictures. However if you've installed or updated any programs between the restore date and now you'll need to repeat the process).
Chris
PS: If you've recently installed any new software or hardware, that might be the cause of the problem anyway. Uninstall anything which you regard as 'suspect'.