You need to dig up your old windows installation cd and the activation key.
Then you just need to install windows on the new pc.
However there is a problem with that if you have a SATA drive as the windows xp installation cd doesn't have drivers for it.
If you have a SATA drive (which is almost guaranteed ion a new pc) you need to follow this guide: http://news.softpedia.com/news/Install-Windows-XP-On-SATA-Without-a-Floppy-F6-47807.shtml
I think it's best to remove your windows installation from your old drive (eg by formatting it, however this will also remove any data like pictures and docs) so it won't start bitching about running the same key on 2 machines.