Need more information...has this happened suddenly or been a gradual decline?.......do you know WHY you can't defrag your drive (i.e. is it because you cant run the program or because you get a warning..if so what is it?) ....how much space do you have on your hard drive.....do you know how many processes you have running?

Theres quite a lot that can influence a PCs performance so its a bit difficult to know for sure without more info

nexus.