During the installation of Service Pack 1 (SP1) for Microsoft Windows Vista, the program makes a backup copy of all of the files that it plans to overwrite.  The installer will not remove these files when it is finished, it leaves them on the hard drive.  The reason is such that you can have the option to rollback the service pack in the event of a catastrophe.  I'm not sure how much space this takes up, but I suspect that it is similar to the size of the service pack file once it is uncompressed.  The compressed file size is 450Mb for the 32-bit version of Windows Vista, and 750Mb for the 64-bit version.  The uncompressed files likely equate to 1-2Gb of hard drive space.  That is a significant amount.

I've been running SP1 for a couple of weeks now and I am confident that this update is stable, so it's about time to remove those backup files and free up some much needed space on my hard drive.  Let me tell you, for the first time in the history of the company, Microsoft couldn't have made it any easier to do.  There is pretty much one step, however I have broken it down for those of you who aren't as savvy with the computer.

1. Click on the 'Start' button, then click 'All Programs', then 'Accessories', then 'Command Prompt'
2. At the command prompt type vsp1cln.exe and press <Enter>
3. When prompted by the User Access Control (UAC), click 'Continue'
4. That's it.  You should see something similar to the screen below.

*** Please note that following the above steps will make Service Pack 1 a permanent fixture on your computer.  You will be unable to roll it back, so do so at your own risk.  I take no responsibility for impacts of the process outlined above.  I can tell you that I have done this and have experienced no negative results.