Installing XP on a Compaq Presario

I recently bought a very cheap Compaq computer that i considered an okay deal, 300 hundred bucks for a 2.7 Ghz processor, cd burner, a gig of ram, and an allright cd burner. The problem I have is that this pc only came with a six month trial of office works, which is a problem considering a bought it solely to be a work pc. I have an old copy of xp and a copy of microsoft office, and considering the smaller amount of resources required to use xp it seems that my best option would be to install those. I have built a pc from scratch before, so i know how to go about installing xp, but i am worried about unforeseen consequences if i try to delete the vista partition and make a new xp partition. i have no experience working with compaqs, so any advice regarding what partitions are safe to delete, how to keep the computer from reformatting to vista (the restore info is stored in the D drive in my particular model), or just general dos and don'ts would be very much appreciated.
Jack