You can look up all the entries in Task Manager>Processes tab on
this site. It's a huge alphabetized list. I've never failed to find what I'm looking for in that list.
To shut down XP's unecessary services, the guide on
Black Viper's site is a good one. Use the settings in the Safe column.
Add
Spybot to your arsenal of maintenance tools. It tends to find things Ad-Aware doesn't and vice versa. Just be sure to use the "check for updates" feature in both Ad-Aware and Spybot each time you use them.
Stop programs from loading on startup (most put an icon in the system tray) but which you can start later if you need them.
It's best to look in each program's options/preferences/settings (whatever terminology that program uses) for the option to disable it's loading on startup. It's often hard to find (on purpose, they want their program in your face at all times

) so be persistent.
For those programs that do not have the above option, go to the Start Menu, click on Run, type in "msconfig" (without the quotes), click OK. Go to the Startup tab. Go down the list and uncheck those things you
know don't need to load on startup....which you determined by using the list I posted a link to in the first paragraph.