Why has my computer suddenly slowed?

I recently downloaded a new program and shortly afterward, my computer started top slow down horribly. Whenever I type, there is about a 1-minute lag time. Programs will start, stop responding, and I will close them after 15 minutes of waiting. Internet loading takes forever. (For example, I opened my web browser 20 minutes ago and am just now starting to type this message. I deleteded the new program, along with a few other things I had downloaded over the past week, and also all the nonessential pictures, notes, and miscellanea I have been collecting for months. That didn't work. I tried to run a virus scan, but after 24 hours, it had only gone through about 400 files of my computer. Any ideas? At\ all? Anyone?

(P.S.: When I tried to empty my recycle bin, I accidentally deleted it. How do I bring it back?)

(P.P.S: I'm running Vista, if it means anything)

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