Hard discs don't last forever! 5 years is pretty good. As fo3nix says, make a backup of anything important then do two things (at least). Visit this site:-
http://www.ultimatebootcd.com/
and make a bootable CD (it has rescued two friends' computers so far).
Then see if you know how to get to a 'Command Prompt' (Start/Programs/Accessories/Command Prompt) and type in CHKDSK c: /f (there's a space after CHKDSK and a space after c:). You may be told that Windows is using a file and needs to do this the next time the computer starts up. DO NOT RESTART! - make sure that you say OK/Yes to this and then SHUT DOWN! When you switch on, the computer will then check your hard disk and attempt to fix any problems. If you are not satisfied, do it again. In both cases it may take quite a long time - be patient!