I've had this happen multiple times, and for me, every time it was a corrupt user profile. You can google it, but the fix was something like creating a new user, then copying over everything from the old profile to the new profile, and then just use the new profile.
Thanks for all of your advice, I feel much more confident now that I know it is a recognised glitch. Now all I need to do is fix it, since I imagine you folks are comprised of programmers etc... I should warn you that I'm not too au-fait on this type of thing. My query is more accurately worded as:
How me make computer good?
The suggestion that Glass Knife made seems to be pretty good, is anyone able to provide a link to a guide for that process? I'd gladly google it myself, but I'm a little wary of finding a guide for the wrong process and potentially worsening things?
Cheers for all your help so far - second to none!