How can I figure out what process is constantly accessing my hard drive in Windows XP?
With task manager you can view which processes are doing the most I/O.
Go to the View menu -> Select Columns...
Tick off I/O Read Bytes and I/O Write Bytes
These should help you identify the offending process.
You're about to the joys of SysInternals (sysinternals.com will redirect to an MS site). Start with filemon, regmon, procmon, and tcpview, and take it from there.
Check out this useful utility to tell you what is going on.