Apple iSight and most all other cameras you have working on Mac OS X are now shared with Windows virtual machines.
They should be available and work fine in Windows without any additional settings.
NOTE: If you prefer to keep your cameras separate between Mac OS and Windows virtual machine, please go to your virtual machine's Configuration -> Hardware -> USB & Bluetooth (USB Devices in Parallels esktop 7) and remove a checkmark from webcams sharing:
in Parallels Desktop 8 and later:
in Parallels Desktop 7: