If your tower did not come with a restore CD, it might have a hidden recovery partition. Many OEM systems like your IBM do not come with the actual software CDs. The come with a restore image that wipes the hard drive and takes you back to the way the system was when you first opened the box.
As to Windows XP, there have been 3 releases. Often referred to as Gold, with Service Pack 1, and with Service Pack 2.
There are 4 versions of XP. Home, Professional, Media Center, and a 64 bit version of Professional.
For all of the above, there are various iterations and permutations.
Full retail.
Retail Upgrade.
OEM upgrade. (this version is very limited as to how many copies were distributed)
OEM.
Corporate Edition.
Finally, within each of the above are distributions. On the OEM level, you cannot use a Key on an IBM system with a CD from a Dell. On all levels, on a periodic basis, Microsoft creates a new algorithm for the key code and starts a new run of CDs. You cannot mix key codes and CDs from different batches.
Do I have you confused enough yet?
Chas