Due to the risk of partition editing and the damange it can do, please do NOT take this advice until another member says that it will or will not work.
I personally have modified my partitions to install Linux in this way many times before, and have had no problems. Still, verification can be extremely useful.
What version of Linux?
Most versions of Linux (Ubuntu is my favorite that can do this ) have a built in partition editor with the installer.
You should install Vista before you install Linux. Did you already install Vista?
Is Vista running?
If Vista is working correctly, it does have a built in disk manager.
Press the Start button, type:
Computer Management
in that program, on the left hand menu, click: "Disk Management"
Right click your partition, and click "Shrink Volume:
Warning:
Vista has a relatively safe method of modifying the partitions without even needing to reboot, HOWEVER, there are still possibilities of things going completely wrong. Backing up is a very vital step before any partition edit.
Shrink the volume down. 20480MB is 20GB.
And theres a new, non-formatted partition waiting for you.
You can then boot up in Linux (from here, I can only speak for Ubuntu), and run the installer. Select the newly created 20GB partition.
