Let us know what AV software you are running, if you are using any of the backup programs that are mentioned above.
If push comes to shove then i would reinstall it and just install the programs you would usually do without updating the OS. Then after a few weeks if it starts running slow again you know its installed software thats causing the problem and not Windows Update. Out of curiosity what programs run at start up? If you need a quick way of looking the at the startup time programs goto
www.mlin.net and download startup control panel from there. It will also tell you what programs are called by the registry at startup.
When you find out could you post the list here? It may help to see where the problem is