"I have a bunch of movie downloads on my computer. They look fine on my computer but lousy on my HDTV. Can I convert these movies to high definition with my computer so they look better on my big screen TV?"
While you certainly can convert a standard definition movie to a size that's native to your HDTV, there is no magic trick for converting a movie optimized for standard definition screens to high definition. When a movie is compressed for viewing at a specific resolution (like DVD quality at 720x480, for instance) the image data in that file is optimized for viewing at that size or smaller. Stretching that image to a larger dimension typically makes it look worse, although there are some pretty good hardware scalars built into DVD players that can do a decent job of making DVDs look better at larger resolutions.
My recommendation is not to convert the movie to a HD resolution, but to alter the way you view the file on the screen. The correct thing to do when you have a video that's optimized for a specific resolution is to view the video at that resolution, on whatever screen you use for viewing. Most HDTVs have modes that help you do this. If you downloaded a movie from the Internet that's 640x480, which is a fairly common size, the optimal way to view it on your big screen is to set the television in 4:3 mode or 4:3 with pillarbox mode. These modes typically have black bars on the left and right sides of the video, reducing the percentage of the screen used for playback of the video. This trick also works for television shows not optimized for your HD screen.
