In Win98 you should be safe killing everything except explorer and systray in the task manager. The Win98 task manager isn't that good and doesn't show you everything that's running. Another example of MS protecting you from yourself. I use a 3rd party task manager that shows more but unless you really know what you're doing you can and probably will crash the machine. As long as you don't change any of the standard startup processes it's no big deal - just reboot.
The XP task manager shows more running processes. You can find out what they do here
http://www.answersthatwork.com/Tasklist ... sklist.htm and make your own decisions.
If you want to tweak things even more get Custom Startup here
http://gboban.hypermart.net/csup/ ($10) which will allow you to define several different startup profiles (one for gaming, one for surfing, etc.). I don't know whether this works with XP, though.