It is probably an old tipe, but I have just configured my Windows XP Parallels VMs to use my MacBook buit-in iSight Web Cam using the driver that you can find here:
The site states that it does not work with Parallels, but I am using the release Build 3170 RC3 and I do not have any issue.
Start your VM
Download the Driver & Unip zip
Grab the Built-In iSight using the Devices > USB menu
Point the Windows Devices Manager Installer to the location of the Unzipped folder
and you are done...