If you don't have the info on the serial number anywhere
Belarc Advisor will be able to tell you as much info as there is (as well as loads of other stuff)
In Control Panel, go to Administrative Tools, Computer Management, then in the explorer-type window choose Storage, Disk Management, and click on the entry for your hard disk.
If the panel at the bottom shows two partitions (eg a 2.5GB or so marked FAT32 or EISA plus the actual C: drive) then chances are the smaller one is another partition with an 'image' of the original XP instal.
Depending on how its set up, chances are that F10 will take you into a screen where you can do a restore of just the XP image without effecting installed programs/My Documents etc (though personally I'd still back up to an external drive first

) or a whole 'back to the original setup' restore.
Of course, you'll then have to reinstall SP2, all the windows hotfixes since the PC left the assembly line, etc.