|Most of the time that's done with a pretty high quality scanner. There's some info here:|
Those turned up first in a convert slides digital google search.
If you have a good scanner you could try that. Usually you need an adapter for it work with slides. Or you can try the other methods suggested such as taking a high quality photo of the projected slide. The good thing is you'll still have the slide so you can always try another method.
But unless you're dealing with a huge number of slides it's probably going to be cheaper to have it done professionally. I'll bet the photo shop at your local wal mart could do it.