First, determine which programs are crucial to you by trial and error and guessing.
Press the CTRL/ALT/DEL keys to get the Close Program Box
Stop one program from running, then use your computer for a while and see if something is amiss or you can't do something you want.
Repeat with each program until you have determined what you need and what you don't.
Basically, though, the only things that have to be absolutely referenced in the Close Program box are Explorer and Systray.
Once you have established which programs you don't want loading automatically do the following if you have Windows
98/ME/XP:
Go to Start/Run
In the Open box, type "msconfig" without the quotes, then press Enter.
That will start the System Configuration Utility,
When the System Configuration Utility comes up, click on the Startup tab
Go through the list of programs. When you find a program that is being loaded automatically against your wishes, click on the line or on the box to the left of the reference to remove the checkmark.