When you use Windows 7 instead of Windows XP, you might need a new virtual CD tool which can open disk image file such as .iso file.
By now the best free tool is Virtual CloneDrive, which you can download it from their official site down load page here.
Note: The last one is the free one which we are talking about:
Virtual CloneDrive (Freeware) 220.127.116.11 2011-03-06 1551 kB
So the exact download address is this one.
After you install, every time when you right click someone .iso image file, you will see the mount image file menu item as the following: