Please help: how can I get rid of trial version

I have a desktop with win xp 64 bit and recently got problem so I reformatted and couldn't find its windows install CD. I find a trial version vista and installed but only with 30 days. Could you tell me how to get my windows as normal without 30 day reinstall? Any win xp 64bit windows installation to download? Does any suggestion if have? Thanks you very much