How to disable sound in certain applications?

From Windows Vista onwards, you can use the Volume Mixer application (double-click on the volume icon in the system tray) to control the sound level of individual applications.

Here's a sample screenshot of it:

You can also run the program by typing sndvol from the Run prompt.

It should work for what you would like to do.

Windows 7 - in the System Tray, double click on the Sound icon and then click Mixer. You should then see a list of your running apps. Click on the speaker icon for the individual app to disable sound.