Ok. I thought you were talking about recording from the speaker jack from an amp. The speaker jack from your computer isn't going to output enough signal to blow your card, as it's not amplified above line level.
So, yes, you can do this, but you'll need to make sure that you select "line in" on the recording properties of your windows mixer, and that the line in, wave, and volume control levels are set properly.
An easier way to do this would be to download a freeware app that records the output of your sound card internally. There are a bunch available. I used to have one ...can't remember the name. It'd be a lot easier. Alternatively, if your windows mixer recording properties has a "Wave Output" or "Stereo Mix" option, you can just select that and record with your MusicMatch.