Post#12 (get the correct version) + Post#28 (link to how-to Repair Install)
Before doing this, get a KeyFinder to get your OEM key. There's also a file "yadayada.XRM-MS" that needs to be used in conjunction. Read this
The theory here is that using LEGITIMATE methods to Repair Install + Re-activate (using YOUR Key + xrm-ms file) with a RETAIL DVD Image (the exact same one as yours) should get you up to SP1 directly
(the Image is pre-integrated) and (in essence) "blow away" any pre-SP1 "fixes" installed (no longer needed).
OEM DVD's are basically Retail DVD's with extra "goodies" (maybe) and the said file with the key "embedded" (sometimes) and OEM Logo (etc etc).
This information is widely available and is only good for OEM Pre-Installed PC's. There's even a thread on MSFN
on how to create an OEM DVD without the extra "goodies" (that you may or may not want) plus lots of other valuable info. I still highly recommend getting the DVD(s) from the Manufacturer.
Final note - For all we know, you might actually have a "Restore To Factory" WIM-image on a "hidden" partition (you may not want to use that except as a last resort - puts it back to when you unpacked and hooked it up i.e. back to square one).