To identify and close the unwanted startups:
click on Start > Run
type msconfig > click OK
You should now see the System Configuration utility
Under the General tab, choose Selective Startup

Under the Startup tab (far right) you will find a checklist of every stupid little RAMsucking utility, agent, and Systray icon that starts up with Windows.

The only ones that you must leave checked for Windows to operate are:
System Tray (systray) and Scan Registry (scanreg). You can safely remove all other checkmarks, except for things you may actually want to leave running like your anti-virus software. To do the defrag, though, I suggest you disable all of them. EXCEPT systray & scanreg, that's the only rule.

You will find the system booting a lot faster without all that junk in there, and you should be able to proceed with the defrag without a lot of interference. (I am not disagreeing with Jon's suggestions, this is just another approach, which will continue to benefit you after your next reboot).

If by some chance you find after rebooting that you can't find something that was actually useful to you (and you don't know which startup item it was), you can go back to your original bloated startup menu by going into MSCONFIG again and selecting Normal Startup.

W-O