Clean and LeanI used to wonder this myself until I realized something: It really depends on what you want, and what you can support.
I personally have an XP pro machine which cleans itself each night while I sleep. Total processes: 33 when running with most of my pre launch stuff running. OpenOffice, Google desktop search, Antivirus, sound control (damn you AC97 I love your 5.1, but why do I need such a large app to control you?!), wireless apps (scanner, plotter, and controller), Firefox, and Thunderbird, and maybe on average one more app. If I am using anything else (read: Gaming), I close down most of my apps to try and keep my computer from over heating. Coincidentally I'm actually posting about heat right now in a new thread.
So I use around 30-35 running a (what I consider) a rather barebones/clean machine. But my father insists on running his computer with around 60-70 running. He complains about slow load times, but he doesn't want to lose the fast load time of any of the obscure programs he might load. So my advice: Go through the list and find what you don't use. You can generally get rid of it in some way. But unless it is seriously eating up your ram then don't worry about it.
Sager 4080 (Clevo D400V), Pentium 4 2.8 GHz w/ HT, 1 Gig 3200 Ram, Radeon 9600 Moblile, Tall custom built pads to lift for air flow, XP Pro.