XP running slow

I have a Dell Dimension 4500 running on Windows XP service pack 2.  I bought it back in September 2002, yeah I know it's old...in under a month the original hard drive completely crashed and Dell sent out a tech to install a new hard drive and reprogram the computer.  Since then I have had to reformat and reprogram the computer at least 3 times now because of slow performance.  I can not figure out how to resolve this problem.  I have tried virus scans, spyware scans and removing most programs from the computer.  My CPU usage goes crazy and the computer just stops at times.  Usually system idle will use 98% of the CPU but then other times explorer (not internet explorer) will use a large part of the CPU usage.  The busy light is almost constantly solid.  The only suggestions I have gotten is that my hard drive is going bad (again) or another friend suggested a glitch between the Dell computers and Windows XP service pack 2.  Does anyone have any suggestions or fixes for this problem?  I am ready to just throw this computer out the window pretty soon, which doesn't help a student needing to do school work and business work at home.  Any help would be much appreciated.  Thank you in advance!