Hmm, thats a good question. I seem to remember that some program that I have downloaded (or otherwise installed) has a feature that lets you do it. Haven't tried it yet to see if its any good though. If it isn't, then I shall just try out some freeware programs until I find a good one. If you happen to know a good program that'd be great!
As you can see, I haven't thought this through all that well - but I wanted a new challenge to keep me occupied for the next week...