Channel Labs offers a step-by-step guide for providing your customers with Linux on Windows Vista PCs.
Booting Vista Out and Linux Up - 'Shrink Volume'
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To shrink the volume, simply right-click on the C: drive partition and select “shrink volume.” Vista will calculate the amount the volume size can be reduced and provide you with a dialog box to control that amount. A good rule of thumb is to free up 40GB