Failing Uninstall Service Pack 3 - Computer Doesn't Start

My computer is Toshiba Satellite A215, with Windows XP home edition. I had Service Pack 2, but about a week ago, I updated to Service Pack 3. After installing Service Pack 3, the whole computer slowed down and started acting weird. The speaker started to make noise, it started to show error messages often, the internet does not work smoothly, spyware & virus search take 10 hour or more and never finish (it used to take only a few hours), etc.

So, I wanted to put the Service Pack 2 back to my computer. I tried uninstalling SP 3. But it failed uninstalling and I got a message says something like "couldn't uninstall SP3". I thought it might let me uninstall if I restart the computer and I tried to restart, and it never start again. This blue screen with a message, something like "autochk program not found-autochk skipped" comes up and then it goes back to the Windows LOGO screen and keeps doing the same thing. I read about some similar problems, and tried a few things, like starting with safe mode or last setting that worked, etc, but they do not seem to work....