computer is so slow

My computer takes forever to boot up. It takes ten minutes to boot up, load Outlook Express and load LJ in a Firefox window. And then for the next ten minutes, it crunches furiously, going through the rest of its start-up routine, so everything runs slowly then too. That's twenty minutes to start up! What gives?

What's going on in the background? Does every program I have send out for updates upon start-up?