It should be, as SP1 was released a while back... but I believe there is a little thing on the left side, under the certificate of authenticity and the think that either says "upgrade" or "full", it says the version. (mine says "version 2002", and I believe the SP1's say "version SP1" or "version2004 SP1" perhaps.
My win xp pro has sp1 in it, it came that way....and it also happens to block out most of the 1st generation "corp" cd keys. This was from last year when intel was giving xp pro away, along with the purchase of mb and processor from retail edge.
So if you have the corp. ed. you need to chng the cd key prior to sp1 install otherwise it wont work.