Hi there,
Computer specs
Athlon XP 3200
1GB RAM consisting of (2x256 and 512)
2 Western Digital drives 40GB (OS), 200GB
Windows XP
I can't remember the motherboard of hand.
Lately the computer has intermittedly had 100% usage occasionally. Sometimes the computer would crash on login as well. Then a day or two ago the computer crashed when loading windows. The computer wouldn't boot in safe mode.
Then proceeded to use recovery console. Chkdsk caused the system to crash. Western digital diagnostic CD caused the computer to crash. Attempted to format the OS drive and reinstall. After a painstaking amount of time it did install but crashes or at least hangs forever when logging in.
Have tried replacing the hard drive IDE cable and socket. Tried replacing OS drive. Disconnecting the 2nd drive didn't help. Tried taking out memory chips still problems.
The computer does login eventually now but has 100% cpu usage on fresh install. The usage does go down slightly if nothing is done however the computer takes an age to do anything.
When the computer was not booting I did use BIOS fail safe settings orginally. Stupid question I am sure CPUs can't be underclocked can they. The motherboard apparently automatically overclocks the CPU and I think this cause the computer to crash once or twice months ago.
Prior to the computer not working I don't think the computer was opened in months.
Any and all suggestions welcome please :(

A couple of things come to mind, have you run scans for infections? Bear in mind that reformatting may not delete some viruses, you will need to either get rid of them by using anti virus, anti spyware...or wipe the hdd and start fresh that way. Reformatting only deletes the addresses, wiping the drive will over write everything.

Did you defragment the hdd after installing the OS?