Windows Vista sidebar, is an area at the side of the desktop that you can place various tools and displays
http://www.microsoft.com/windows/products/windowsvista/features/details/sidebargadgets.mspx
It will make a difference if you're short of memory. I've removed it because I didn't really use or need it.
"How do you determine what things to let run?"
You decide what you want to run at startup. Running programs at startup allows them to startup faster when you want to run them. Not all programs need to, or should run at startup. As ever with computers, if you're not sure, leave it alone.
There are a few ways to adjust what runs at startup. First off, go into the programs options/settings and see if there's an option to disable it running at startup. Failing that, have a look in START, ALL PROGRAMS, STARTUP and see if the program is listed there - if it is you can remove it.
If it's not there, you can go into START, RUN, type MSCONFIG then OK. Go into startup and see what's set to start with windows - uncheck what you don't need at startup. (Quicktime is one I always disable.)
If you have something that is persistantly running and you just want to delete it but can't; download a program called hijackthis
http://www.majorgeeks.com/download3155.html
You need to be careful using this software though - only remove stuff you are sure about. This has got me out of countless problems with windows XP, not used it much with Vista though.[/i]