Windows XP question

Well, I'm building a new PC, and I have all the parts selected and everything, but the only thing that remains is the OS. I went around newegg, and saw that a windows home xp with sp2 is about $189, and with all new pc, im gonna be a little short on money. So I was wondering if I could use my old computers OS, its a Windows Home XP with SP1, with the product unregistered online. The only problem is that I don't have the installation disc, but I can get one somewhere if I can use it. So can I or can I not still use the it? If I can, do I have to find the exact type thats installed on my old pc?

If the XP Home on your old computer is a retail version and you remove it from your old computer than you can transfer it legally to your new computer. If on the other hand the copy you currently have is an OEM version (came with the PC or otherwise) or you don't want to remove it from your old PC then you'll need to buy a new copy of the OS if you want to stay legit.