I finally figured out the trick to getting the Vista SP1 upgrade to show on Microsoft Update: You need to make sure your Thinkpad has updated drivers installed before the SP1 update will show.
I made the mistake of downloading the manual installer for SP1 and installing it without doing the driver update prerequisites. End result: My x60t wouldn't boot and I had to use the Lenovo Vista boot disk to go back to a restore point. Thank God that worked!
Through trial and error I found these drivers are the ones that need to be updated prior to being able to install Vista SP1 using Windows Update:
Here's where to get them:
- UPEK TouchChip Fingerprint driver: http://www.upek.com/support/downloads/drivers/windows.asp
- Intel Mobile 945 Express Chipset Driver: http://downloadcenter.intel.com/Detail_Desc.aspx?agr=N&ProductID=2301&DwnldID=15829&strOSs=All&OSFullName=All%20Operating%20Systems&lang=eng
- SoundMax Integrated Digital HD Audio driver: This one will appear for you in Microsoft Update.
Once you update these three drivers, Vista SP1 will show in your downloads when you go to Microsoft Update.
