How do I know what to delete/uninstall/disable?

I want my pc to run faster. So, if I were to clean it up and/or change the startup settings, etc....how do I know which programs are really necessary to have running/keep? i.e. when I do run:msconfig to check startup...sometimes it tells me it's from Microsoft, but I don't know what it is. And sometimes, when I try to add/remove programs...I don't recognize the names. How do I know if I really need it?

How do I back everything up because I do have the disk for windows xp that came with the pc?

Windows needs to be refreshed every few months to run well. Typically I re-install Windows every 6 months. I would not recommend trying to uninstall programs for performance. You won't see any improvement unless your hard disk is completely full. Best bet is to backup what you need and reinstall. Trust me, once you get in the habit of reformatting every few months you will be very pleased with the performance gains.

use this
http://www.sysinfo.org/startuplist.php
type in the name of the startup item and take it from there.

If you are not familiar with the software process, the move will be risky. You will not know what will you do to your operating system. Plus, don't know what type of software and hardware spec you have as well as how fast you want your PC to run. One of the safest way to speed it up is to upgrade your RAM.