"As a little background, my R2004 license permitted one update."
Do you have a subscription for AutoCAD? If so you would have every version since 2004. If not, your license didn't give you an automatic upgrade to 2005 since Autodesk does not do that.=A0
"I then was permitted to download the next version, during that period, which I immediately put to this CD. =A0This may be the first time I've ever tried to install from it."
It sounds to me like you downloaded some bootleg copy of AutoCAD from a site other than Autodesk. As I stated before, unless your 2004 license is on subscription you are not entitled to an automatic upgrade to 2005. And if you have a current subscription you would have very version up through 2009. I have been installing and supporting AutoCAD since version 12. I have never seen an executable from Autodesk, whether on disk or downloaded from the website, that is corrupted with a virus. Your machine probably already had the virus and when you ran the install it became apparent.
As for Vista, what would lead you to belive that 2004 would work if
2005 is incompatible? The only versions that are compatible with Vista are 2007, with the service pack, 2008, and 2009.