I reinstalled SP2 from the internet but doesn't seem to have made any difference

With XP, you can not run the sfc command to verify the system files without the Windows XP SP2 Installation CD. Just reinstalling SP2 is not the same thing.

Where can I get the CD

This is one place - eBay - Microsoft windows xp pro sp2 FULL, Software, Computers Networking items on eBay.com=, but I'm sure you can buy on where you live. However, this will give you an idea of the prices.

Could I get one from the place where I bought the comp?

I would think so, but I do not know where you bought your computer. Ask them. The worse that can happen is they tell you they don't stock it. Right?

Just be sure that it is a Microsoft Windows XP Professional SP2 FULL version.

or you could always borrow it off a friend if they happen to have an xp installation CD, just to do the file check