I was having trouble finding a good Vista-compatible virtual drive utility to mount CD/DVD images, so I thought I'd share the result of my quest.  SlySoft's Virtual CloneDrive is completely free and without ad/spyware.  The system requirements specify Windows 98/98SE/ME/2000/XP, but it works great on Vista, as well.

Virtual CloneDrive works and behaves just like a physical CD/DVD drive, however it exists only virtually. Image files generated with CloneDVD or CloneCD can be mounted onto a virtual drive from your hard-disk or from a network drive and used in the same manner as inserting them into a normal CD/DVD drive.

Probably the best virtual drive software, Virtual CloneDrive allows you to enjoy the freedom of a virtual drive and is completely free.