reformating

So I have computer laying around and it's pretty infested with malware, spyware, ect.. So I'm looking to reinstall xp. I tried once and it said that the version on there was a more recent version of xp and wouldn't let me reinstall,so the first thing I thought is to do a reformat of the HD and do it that way, but I thought I should check here just in case there was an easier way.
So any thoughts or ideas or should I got for the format.

Subtle troll is subtle.

How do you want to get rid of the malware if you don't format?