The problem with a soundboard in a club is that it may not have all the band going through the PA--particularly the drums, but also possibly the bass and guitar. You'll get a recording with the singer WAY UP FRONT.
There is free stereo recording software, Audacity, for laptops. It's stereo, and will record from your laptop.
The mic input on your laptop is almost guaranteed to be mono. Stereo is probably better.
I don't know anything about the gadget recommended by Chris Church, but he's a serious audio guy, so it's worth checking out. A gadget I have used is the Griffin iMic, about $25-30, an external soundcard that has stereo inputs and connects to USB. Despite the i-prefix, it works with Windows too. You'd connect it via USB, go into Audacity settings to recognize it, and you'd get clear two-channel recordings. It has a 1/8" mic plug, no XLR.