Amazon is still offering Windows XP Home, Pro, and Upgrade.
The Upgrade edition upgrades your legacy (98/ME, possibly 95?) to XP Home + SP2.  There’s no upgrade edition to make the jump up to Pro, you have to buy it outright. 

Here are the relevant links..

Windows XP Pro
Windows XP Home
Windows XP Home Upgrade 

Originally posted 2009-03-26 09:00:42.