What Processes are Necessary for Windows XP?

Brickwall asks: "I studied electrical engineering in university (30 years ago, mind!), so I'm not completely stupid about computers. However, I have searched and searched, and been unable to find an answer to this question: if you start up Windows XP from scratch, what processes should be running? I have some P2P software running, so I know I'll have to shut that down, plus my spyware protection, anti-virus software, etc. But what should be left running? Is this documented somewhere that I've been unable to find?"