Hi RigbyBaffers,
Have a look at this article:
http://support.microsoft.com/kb/307545
If that doesn't work, I'd get an external HD case, take your HD out and put it in that, recover any data you want to keep on another computer, put the HD back into your original machine and reinstall Windows.
Assuming your data isn't encrypted and there's no hardware problems, that is a pretty definite fix
(despite being a bit difficult).
Let me know if you need any clarification,
Chris Fry
Canberra, Australia