erasing windows xp and starting over

My computer has a lot of spyware and malware I wanted to erase my hard drive and reinstall xp over again I don't have the original disk that came with the computer. I read if you reinstall a new xp disc it would automatically format the hard drive or that you could buy a recovery disk and run it What would be the best and m least expensive way to go?

Do you still have your product key?
Should be stuck near the back of your computer, or screen.

I found the product key. What next? This is an old Compaq Presario that my sister gave me so I thought I would try to get it working again. There is also a card slid in the back with a phone jack in it would you happen to know what that could be for? Thank for the response Jeff

Your computer will most probably have a recovery partition. Follow the instructions here to perform a recovery:

http://www.geekpolice.net/tutorials-guides-f13/hp-compaq-system-recovery-hotkey-instructions-hp-pc-system-recovery-t7662.htm

this is a desktop not a laptop does that make a difference?

Hello,

It doesn't.