I've made repeated attempts to install Service Pack 1 (KB976932) and I.E. 9. Each time the download is successful and the update progress window says the update was successful, but when I reboot the configuration process stops unsuccessfully and Windows reverts to its pre-update state. Update History indicates all failed attempts.
I have applied the update repair utility offered by MS, I have run WindowsUpdateDig.cab, I've completely scanned my computer for malware (avast! and SuperAntiSpuware) and cleaned anything that was found. Some of the update attempts have actually left Windows unbootable and I've had to revert manually to an earlier restore point just to get back into the system.
The error code in the Windows Update History is 80073AA2. I've been through all the suggested fixes that come up from "Get help with this error." I don't know if there are other logs that would offer more detailed intications of what's going wrong.
A couple of other things of note - since my last attempt at SP1, I have been able to successfully apply a definition update for Windows Defender - KB915597. I have not, however, been able to get the update for User-Mode Driver Framework KB2685813 to install - it has failed several times in the same way the SP1 has. So this seems to be a selective problem.
Any help will be appreciated.