In a bid to be both helpful and sticky, most Windows software seems to install code to be run when Windows starts up. My laptop had about 25 different executables set to run at boot time. Spybot has a great "System Startup" panel that shows all of the startup entries, and lets you disable them individually.
Part of the information shown about each entry is a quick description of what it is, where it came from, why it got put there, and whether it can be safely disabled. Many of these entries are credited to Paul Collins' Startup List, which is one of those reference works only the web could have produced: obsessive, helpful, authoritative, and mind-bogglingly detailed. Its utility is limited to those times when you need to identify a mysterious bit of flora from your invisible Windows ecosystem, but when you need it, you really need it, and it delivers.
Using Spybot and Paul's list, I disabled more than half of my sneaky Windows startups. My machine boots ever so much faster, and I don't even have the "show me more" button on my system tray any more, because I only have icons there that I actually want to see. Bliss.