Running Until Automatically Installed 8 Updates Before Shutting Down

I have a Dell Dimension 4700 with Windows XP. It was running fine until last night, when it automatically installed 8 updates before shutting down. This morning, there is a problem. I turn on the computer. It works fine for about 2 minutes, then slows down and won't respond for at least 20 seconds to anything. The fan is running loud as though it is "thinking" about something or working really hard even though I'm not doing anything out of the ordinary. If I want to shut down, I have to do it manually. Then, when I restart, it works for 2 minutes and slows again. In those first 2 minutes, I tried checking my email, using a couple programs, etc. and they work fine until it decides to slow down. I have restarted it about 5 times with the same results every time.