Is it worth replacing the hard drive and starting over.
There are many things you can do it depends on how extreme you want to go.
Wipe the drive and a clean install of windows or even try running an old machine on Linux.
Some suggestions to try first:-
a) Clear out all temporay files and folders -- use Crap Cleaner click here
b) Scan for malware spyware and viruses --Free Anti Spyware :-
Superantispyware click here
Malware removal tool click here
Free Antivirus software
Avast4 click here
AVG antivirus click here
c) Clean the registry -- Free Registry cleaners :-
RegscrubVistaXp click here
Regseeker click here
TweakNow RegCleaner 1.3.2 click here
Easycleaner click here ( Use with care, It advises you to back up the registry first, this is a good idea as it cleans rather aggressively. )
d) Pagefile (Virtual Memory) -- Rght click MY Computer - select propeties - Advanced tab - Performance - advanced tab - Virtual memory click change, you can put the page file on a differnt drive (if you have one), click custom size and set Initial size to one and a half times the amount of memory you have fitted i.e. 512MB memory = set to 768MB, set maximum to double your memory amount i.e. 512MB memory = 1024MB click ok.
If your hard drive is full and there is not enough room for the pagefile this can slow down, freeze or even cause the PC to crash (restart).
e) Cut down on the programs that load at startup -- Start - Run type msconfig - startup tab- untick everything except for firewall, antivirus and antispyware
and the services that run in the background. click here