Alternatively, you can buy a stereo male jack get some wire, connect to the stereo jack normally then just combine both left and right channels to a mono male jack for the amp.
Take note, tho, if you go the hardware route like that. Combining the two channels at the wire like this can occasionally give you degraded audio, and some people also claim that since the streams mix like that you could overload, or burn out, your stereo jack (ie, the jack on your soundcard might stop working). I have personally done my stereo-mono mixing this way from my guitar amp (for recording purposes… to get the guitar on both channels) and I’ve never had a problem like that in all the years I’ve done it. Just thought I’d give you fair warning in case you’re really paranoid about stuff like that.
For a software solution, you might want to check your soundcard’s drivers or your windows settings to see if there’s a “mono output” mode. If you’re on XP in the sound options there’s a variety of ‘speaker configuration’ options. i haven’t got an XP box handy to check, but there might be a mono speaker option there as well. You might also want to check to see if there are winamp / mplayer plugins that will combine the audio channels for you.