[crunch]

He did say it quite well, just left out a few steps.

Here you go:

Click "Start"
click "My Computer"
Now, on the top menu bar (directly under the bar that tells you what you are in) you will see "Tools". Click that. A dropdown menu appears. Click "Folder Options) Now click on the tab that reads "File Type" Give your system a litle time to browse for file extensions.

When it is done, scroll down to your .pk3 extension on the list and click it.

Now click "change", and then choose the program that you want to use to open it.

And there you go.
Kill Ya Later.