[-=[XTC]=-Body Bagger]

When you make the brush for your window, just texture it with a window texture, but don't give it a function. This will stop it being shot out.
Or if you don't want to re make the window, select it and press the "n" key to bring up the entity window and add the following
Key: health
Value: 1000000 or something really high so you would have to spend about a week shotting at it to destroy it swordfight:
Hope this helps
Craig