As others said, you don't have to, Mark. Install WinXP, whatever version you have.
Then either download SP3 "stand-alone" (aka for network installs or for redistribution), a sw package of few hundred megs, and install it (and possibly keep it somewhere for future use), or simply go through Microsoft Update website and follow instructions. SP1 and SP2 are not needed. They will let you chose if you wish to install SP3 or individual updares; chose the first option.
After applying SP3, go to microsoft updates to make sure you also install later hotfixes, and install alla critical ones.

The above is all free.

Also, I would say that registry corruption is caused by either virus or bad disk. In my personal experience, it is 99% bad disk. So I would still check it, if I were you, just to rule it out and avoid future issues.