Completely tied up

I have a DELL 3.2 XPS Gen2 with 120 gig hard drive and XP Pro.
It runs so slow, nothing will start and if it does, will stop responding after 5 to 10 minutes.
The hard drive light continuously goes on for 1 sec and off for 1/2 sec on off.......
System idle process is at 99% CPU.
Tried starting in safe mode - same problems.
Cannot run any anti spam software as it stops responding.
Tried booting from XP disk but it can't find any hard drives to load it's stuff.
My Hard drive is partitioned as C: and F: with most programs on F:. F is not found when I boot to the command line.

Thought of buying a new hard drive and installing as the master and loading XP Pro from scratch and then try to save some of the data files from the infected hard drive.

I'm desperate. Can anyone help.