yes, vista can be installed on I-macs ...but i need to know which version of operating system do you have (whether it's leopard or tiger and version 10.5.2 or earlier)
I say so because new version of Leopard Operating system has a feature called "boot camp"... this is Apple's "bootloader" ... which means it will help you install windows Vista on Apple I-mac .
If you have genuin vista disk handy with you and after you have backed up all the data on the apple machine , this is what you gotta do....
1. Using the Finder, navigate through Applications, Utilities and run Boot Camp Assistant.
2. Click Continue on the Introduction screen.
3. Next, you will need to specify how you want your hard drive
divided up for Windows. A separate partition will be created that
Windows will use. Drag the divider to adjust the size for Windows as
shown below. I recommend at least 15 GB to allow for the OS and some
apps. When you are ready, click Partition and your disk will be
4. It is time to start the Windows installation
Insert your Windows Vista install DVD and click Start Installation.
Your Mac will automatically reboot and boot to the install DVD.
5. Install Windows Vista as you would normally do.
When asked where to install, select the partition called BOOTCAMP. You
will need for format this partition with NTFS.
6. When Windows Vista has installed, remove the Windows Vista
install DVD and insert your OS X 10.5 Leopard install DVD. Run the Boot
Camp installer application and all the Windows drivers for your Apple
hardware will be automatically installed.
7. When you want to switch to OS X or Windows, just reboot and hold down the Option key to select what partition to boot.