Common Windows 7 issue that has nothing to do with Android (this time)
Plug your phone in.
Find it in the device manager (look for the yellow triangle)
Right click, and choose “Update Driver Software …”
Choose “Browse my computer for driver software”
Choose “Let me pick from a list of device drivers on my computer”
Scroll down and pick “portable devices”, click Next
Choose Standard MTP Device > MTP USB Device
When it warns you that things may go tits up, click yes anyway.