If you have a Creative Labs Soundblaster, you may have a Legacy Device which conflicts with MIDI interfaces on the parallel port.

Check in your device manager (Right-click on My Comptuer, go to Properties, Device Manager) and check for "Creative Misc. Devices" OR check in "Sound, Video, and Game controllers" for "Audio SB64 Legacy Device". Double-click any Legacy Device found and checkmark the box "Disable in this Hardware Profile". Restart your computer.

This will not affect the working of your Soundblaster, except for in DOS mode.