Legal OEM versions have all the fuctionality of the retail versions except for the following.
1. OEM does not come in a box.
2. OEM is only licensed for one computer.
3. OEM is not transferable.
4. OEM cannot be used to upgrade.
5. Microsoft does not supply free technical support or installation support for OEM.
Note: Upgrade does not mean update, you will still be able to receive ALL Microsoft updates including any service packs, security and OS updates.
I would make sure you purchase the OEM from a reliable retailer as some sites sell "warez" versions that will only screw you in the end.
Here is a great site to buy from.
Microsoft XP OEM