Slow computer

What is it that makes computers slow? It takes my computer like 5 minutes to shut down everything at night. It gets frustrating when I'm trying to switch from the internet to something else and it takes forever to get the other thing open. Do I need to buy memory for it or something?

I really don't know anything about computers, my husband says it's RAM or something. Maybe I just expect my computer to do too much at one time. I mean right now I have 6 tabs open in firefox, AIM, MSN, one chat, and my thunderbird email... is that too much for a computer?