That's sooo unfair Whiskas
I'm not sure there is much you can do to upgrade your PC, you will need to replace most of it I'm afraid. It's as old as the hills in computer terms
Unless you know of a source of second-hand parts, you will need to buy, at a minimum, new motherboard, processor, RAM and case. You should be able to salvage the hard drive, CD-ROM and floppy drive from your old PC.
An integrated motherboard should cost around £60, the processor about £30 for low end one, about £40 for a decent case and about £30-£60 for memory. You'll also need to find someone to put it all together for you.
Around £200 should get you a half decent PC