I can't really tell you what to turn off because background applications vary on every computer. Some are essential, and some arent. You just kinda need to figure out what you need running or not. I would suggest running "msconfig" from the run line, going to the Startup tab, and unselecting all programs that you don't want to automatically load up when you start Windows. Don't uncheck any boxes that run out of your Windows/System32 folder however, usually those are essential to system processes.
Also, you can disable many default Windows Services that you may not ever use. Do NOT use msconfig for this, it can mess stuff up. Instead, run "services.msc" and go to
http://www.blackviper.com/WinXP/servicecfg.htm to find out which ones you should turn off or not. (Depending on how you use your computer you may or may not want to disable some of these services.)
Lastly, end the task of Explorer right before you log into the game. This will free up several megs of RAM, but will make your desktop icons and Start Menu bar go away, so its usually wise to start the Launchpad before you do this. After you log out from PlanetSide, bring it back by selecting "New Task" from the task manager, and typing in "Explorer".
http://blackviper.com is a great site for tweaks an the like to make your windows run faster and be less cluttered. Personally, I hate having a lot of useless junk running in Windows. If I want it running, I'll start it up. Otherwise, it has no business running on my machine.