hard drive failure or windows issue?

I've got an Insprion 700m that's about 4 years old running XP.     I've recently had a major drop in my computer speed, the hard drive seems to be running a lot and the cpu load is consistently very high.   Programs locking up or giving the "not responding" error and t`hen after a pause will come back.   The inability to have multiple programs open and running, etc.    I play a lot of audio through this and there are blurps in the sound when it's playing as the hard drive indicator light blinks.    It's getting pretty annoying.

I had a nasty virus that got in and thought I had cleared all of that up.    Trojan virus removed, running AVG, spybot, adaware, optimiser.    I've also defragged as well as dumped a bunch of stuff off the hard drive which is now running at 20% free.    No new programs than when it was running at optimum level about a month ago.

Still having issues.   So how can I exactly tell if it's viral or if it's just a matter of the computer getting old and starting to fade?  This seems to have happened awfully quickly.

Therre are also missing windows modules upon boot up .. and I've noticed some paths to shortcuts have disappeared.       I'm not sure if windows is the issue or the hard drive and how to go about reinstalling windows as obviously the version that came with it is 4 years old