I want to transfer a lot of files from one PC to another as there is a fault with the source unit that needs to be fixed 'off-site' (another story). Both PC's are similar spec and use Windows XP Prof. I do not have any wireless equipment so would need a cable connection if that is possible. What is the best way to do this? Do I need any special software/hardware?

Go for the KISS option and use a simple jump/flash drive or external USB hard drive.

What is KISS? Is a jump/flash drive a USB memory stick?

KISS - Keep It SIMPLE, Stupid.

Yep, jump/flash drive is a USB memory stick.
There's some many different names for them I was hedging my bets..

If both machines have ethernet, you can set up a network between them.

If at least one has a auto-midx, then you don't need a crossover cable.

Both can be set for DHCP and then both with get an address on the same network. On number is 254 as seen by ipconfig /all from the cmd prompt.

Goggle how to share files under windows.

It's not as fast as the portable HD option or USB option. Ethernet is probably 100 mb/sec and USB2 is about 470 mb/s without overhead in the protocols.