The only thing I'd changed was that I'd Flashed my BIOS to the latest revision, and reset the BIOS to it's defaults - it may sound like an obvious thing to do, but if you haven't done it, then try it.
I'd probably reinstall 3 times (for varying reasons) before the install the gave me the correct speed - and the installation disk I used was a "new" one (shrink wrapped copy).
If you do a search for my previous post, you'll find that I quoted a Microsoft Knowledge base artical which talks about the problem (well sort of anyway) - I contacted my M/Board manufacturer (MSI) and they confirmed that there was no problem, so it really looks like it could be a configuration problem on your machine - and most likely it's the BIOS... but I could be wrong...