I thought that it was detecting the file on your system and not the one being downloaded...
I assume that the windows update process is downloading a new version of that file to be used with the remainder of the update, etc. and it is that new version that is being detected. The avast virus log viewer should confirm the location of the suspect file, check the avast! Log Viewer (right click the avast 'a' icon), Warning section, this contains information on all avast detections. Report the findings.
Normally I would say you could pause the web shield, which would allow the file to be downloaded (then the standard shield would object but the file is on your system), but because this is a windows update and it is an installation process I wouldn't advise it.
How to get round that would be the problem and I can't really offer a suggestion. I don't use Vista nor 64bit OS, so I can only hope one of the Alwil team can pick up on this topic and see if they can get a copy from another (MS) source.