Flybye - I don't mean to sound carping, but it would really help if we had a bit more info on the PC in question. I think we can assume by now it's not a laptop.
Things like age, make, CPU, memory, BIOS date (very important, because older BIOSes may not cope with large HDDs)[see it briefly on the screen at bootup], current and new HDD size/make, memory, peripherals fitted CD-ROM/Zip/etc.), operating system and so forth.
Cloning a drive isn't difficult (at least with 95/98) and several of us could talk you through step-by-step but if you're unsure 30 quid may be the best answer.