If your computer has a SD card reader, you can use it to download your photos. I've never used my D3000's USB to computer since my computers have SD readers.
I second Itai's comment. If your PC doesn't read SD cards, obtaining a reader is the easier option.
If you still want to persist with using the camera to PC via USB, and go through "pain" (that I am too lazy to do myself with my D3000), please read on...
NB: I would prefer the SD card reader option as I'd prefer to be taking pictures instead of playing with Windows 7 and its Device Manager.
I had a look at the Windows Compatibility Center and it reports that the D3100 is compatible for use with Windows 7.
These statements and questions may come across silly and redundant.
Did you connect the camera before installing the correct drivers? If so, leave the camera disconnected, uninstall the Nikon drivers, and installing the Nikon drivers, and connect the camera.
If you're certain the above doesn't apply, I would try the following:
remove all USB devices (except keyboard and mouse) from the computer,
connect only the camera and see if Windows 7 is able to correctly recognise the device and hopefully download the drivers.
If it doesn't, explore Device Manager, or consult a friend who is very savvy with Windows 7 and using Device Manager or who is patient enough to diagnose drivers.