You will need the Full OEM. I just recently fell into this problem when someone lost my full version CD of XP Home. I had a copy of XP but it was a dell issued but I found that out AFTER the install and the code screen poped up. Windows was somewhat loaded so I just went **** it, lets try to load the Upgrade. Loaded, worked.... but something is very corrupted I found out. Takes a LONG time to boot windows, ~5 minutes and just as long from clicking Shut Down, to get the option to Restart, Shut Down, and Log off.
Don't try it, just get the Full OEM. I was fortunate enough through my work they have an account with www.atomicpark.com and I got Full Windows XP Pro for $132... paid $140 for 2nd day shipping.