Visual Studio 2005 install on Vista: Unknown error copying files to temporary folder

While installing Visual Studio 2005 on Vista Ultimate x64 I got the error "An unknown error occured while copying files to your temporary folder. Setup will now exit."   I have two CD/DVD drives on my machine (HP Media Center m7170n), so I took the setup CD out of the DVD burner drive and put it in the other drive - presto, fixed.

I had this problem before, and couldn't remember what I did to fix it.  Posts on this problem are rare, and none of them document a real fix, only workarounds.  Unfortunately this is not a 'fix' either, but figured I'd post it for 'post'erity.