http://store.yahoo.com/computersyste...-winxppla.html
If you just buy the certificate of authenticity, you can get it for ~$80. If you're a student, you might be able to get the academic version, which might be a bit cheaper. You'll still need an install cd of course. The OEM version, which includes the COA, the CD, and all that good stuff, and probably requires a hardware purchase will run you closer to $150. Hope it helps.