Sure let me help you out for this.
Get yourself a "RCA (male, left and right)-> 3.5 Stereo (male)" jack cable (gold, good quality).
Plug the mixer's LINE OUT to the PC LINE IN
open your sound options in the PC (double click on the small speaker icon next to your clock) and ENABLE line-in in BOTH PLAYBACK AND RECORD modes.
Get yourself Sound Forge, or another WAV editor. Open it, and set the RECORD to 44.1Khz, 16Bit, Stereo.
Set the volume of the mixer to half (NOT max!) and play something.. make sure you can hear your mixer's sound in the PC speaker.
Press RECORD, you will start capturing the mix... do some tests first.. You can save your WAV file on your hard drive and play it anytime you wnat.. burn it on a CD, or even encode it as a MP3.
Make sure it doesn't "clip" (ie get too loud).
If you have problems you know where to ask