The D3100 is not supported by the Nikon SDK for tethered capture to the PC. I was wondering what options I have to capture tethered, I have found some free versions on the web but only "Diyphotobits" worked for me, however not all options are present it this. I haven't tried gPhoto yet although it works only on Linux. TetherPro does not seem to support the D3100.
I wouldn't writing my own software if someone can point me in the right direction.