Is it possible to change the destination of a link based on whether the user has a mac or a PC using javascript?

To give an example: The Apple website allows download of Quicktime but it "knows" whether you are using a mac or a pc and directs you to the relevant page.

Background/Reason for doing this: I have built a website for someone and they have a number of audio and video files on there. Ideally they want it so that if the user is on a mac it will download a quicktime version of the file but if they are on a PC it will download a Windows Media Player file.

Cheers

CHRIS