I use Microsoft Virtual PC 7 to run multiple versions of Windows and Linux on my G5 (for testing web applications). VPC allows the virtual hard drives grow up to 15 GB -- which can consume quite a bit of disk space when you've got a bunch of them. In addition to consuming more disk space, larger VPC drive documents actually run slower (as if it wasn't slow enough already).
VPC allows you to "reclaim unused disk space" using the Virtual Disk Assistant. However, unless the unused disk space has been zeroed, it cannot be reclaimed. In the past, I had been using a variety of Windows disk utilities to zero the unused disk space. Users of VPC know that everything takes a long time in VPC, including zeroing the free space. Today, I discovered a much quicker solution.
WARNING: Before you try this, do yourself a huge favor and back up the VPC files that you are going to work with. Don't assume that it will always work as well as it did for me.
Here's what I did:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060605064908533