Under windows xp (win2k is similar):
Start>>Control Panel>>Administrative Tools>>Services
Right click on the service, and select Properties. Click "Stop" to stop the service you selected, and set the Startup Type to "Disabled".
Now for the more interesting question: which services can be safely turned off? Turns out... it depends on what you're trying to do.
I've disabled everything except the NI service locator (nisvcloc.exe) and the NI configuration manager (nimxs.exe) and LV doesn't complain too much. But then, I don't have any hardware, citadel databases, time servers, PSP servers, or shared variables in use on this computer. YMMV.
Joe Z.