Tested on Vista Business and openSUSE10.3 on 10 June 2008
This step will explain installing Linux (openSUSE10.3) on existin Vista system. I am sure this works on any Linux.
The Vista PC requires to load the Vista Bootloader first, and the next must be Grub, so you need to edit the Vista Bootloader. EasyBCD can easily edit the Vista Bootloader
So install EasyBCD. This is free, and can handle the booting mechanism for Vista. You can download from http://neosmart.net/dl.php?id=1
Change the existing partition size, and make some space for Linux. You MUST resize the partition size by Vista. DONOT resize the partition by Linux (XP was fine, though)! You just need to shrink the existing Vista partition, and create an unused space. Open [Control Panel]-->[Administrative Tools]-->[Compter Management]. Select [Storage]-->[Disk Management], then the volume you want to resize. Right click, and select [Shrink Volume]. If you can not make enough size, then defrag the partition.
Make an unallocated space for Linux by Vista!!! Reboot your Vista and make sure to boot it again.
Instal Linux (openSUSE) into the free space you created by Vista. It is easy to configure if you choose "Base Partition Setup on This Proposal" option, but any can do.
Note that do not resize the Windows Vista partition by the Linux Installer. The partition I created is as follow.
Boot loader must not be installed at MBR (Master Boot Record). I have selected "Boot from Root Partition".
Once the installer reboot the system, you can probably boot only Windows Vista, so you need to edit the Vista Bootloader
Boot Windows Vista, and execute EasyBCD. Initially, you have only Vista entry as,
Click [Add/Remove Entries] button, select [Linux/BSD] tab. Choose the first partition of the Linux. In my case, the swap is the first. Click "Add Entry", and "Save" button.
Click "View Settings". The whole entry became as below.
If you do not use EasyBCD, you need to find the way to acquire the BCD ID anyhow.
When you boot, you can see Windows Boot Manager screen first, because it is at the MBR, not GRUB.
When you choose Linux, you will see GRUB, then you can boot your Linux system. If you choose Windows entry at GRUB, you can go back to Windows Boot Manager again.