My fathers PC is a P4 at about 1.7 GHz.

From a cold boot, it starts to post, but it hangs for upwards of 3.5 minutes at the first line (before memory)

That line looks something like "<CPU 0F12 PATCH ID 0203>

It's not an exact copy of what it says as I'm doing this from memory, but with that as a rough guide, does anybody have any idea on what's wrong and how to fix it?

Check all the cards, memory and cables to see if everything is seated/connected properly. Looks like a loose connection somewhere.
That line you see is probably the BIOS-version.

Check your boot settings within bios, it may be set to boot from lan.

In a strange turn of event's, the computer has corrected it'self, and now boots without a problem.

Thanks for the input guys, at least now I have some sort of idea for when it happens again!