I had an old system laying around (with an MSI N260GTX Twin Frozr OC (216)), so I decided I'd use it for mining.
And since power etc. is incuded in the rent of my student room, I didn't (and probably will not) spend any money setting it up and running it.
This rig averages about 45 Mhash/s. (temp: 59C)

I also leave my PC on mining during the night and when I'm away. And usually also when I'm just browsing or watching videos.
My PC has an MSI N560GTX-Ti Hawk and averages at about 95 Mhash/s. It tends to get a little hot (79C)

I'm using GUIMiner mining at slush's pool and don't have any options set.