Start>RUN>MSCONFIG, and go to startup tab to see what's checked.
Good information for anything you can reasonably see is in http://www.pacs-portal.co.uk/startup_index.htm.
Unchecking, of course, can easily be undone.
Make a list of things you want to uncheck, and post it here for a second opinion, if you like. It depends on your use of the computer also. A firewall running in Startup isn't necessary if you're not regularly connected to the internet. Anti-virus is doubtful as long as you're stand-alone (but necessary as soon as you're connected). So there is no general advice possible.
IBM ThinkPad 365XD
Running Windows 98
My computer has a relatively limited RAM. How can I safely pare down the programs loaded at startup to only those that are really essential and then keep it that way? Thank you