My old post on "What is all this stuff doing on my computer" described how to track down and identify all the processes that start automatically when you log on to Windows. That post continues to get lots of Google traffic, due to a list of all the weird and wonderful executables that get started on a Toshiba Tablet PC, but thankfully the process of identifying them has just gotten a lot easier.
All you need is autoruns.exe, which you can get for free from Sysinternals.com.
The Sysinternals folks have made two huge usability improvements in the latest version of autoruns: it shows a description and publisher for each entry, plus a simple checkbox to disable or re-enable it. So there's no longer any need to use procexp to find process information, or msconfig to experiment with temporarily disabling something — one tool does it all.