My system is a Quad core running 64 bit Vista.  8 Gb ram.  
I have version 8.0b Vegas running now with no problems.  I downloaded the upgrade file for version 8.1 because it is for 64 bit Vista.
The problem I have it it never makes it past the a certain point in the install process and gives me an error message as follows:
"An error occurred attempting to create the directory  C:\Program Files (x86)\Sony\Shared Plug-Ins.  "
First of all I am logged in as Administrator and have verified I have "Full Control" Privileges.  In addition, I was able to upgrade the DVD Architecture 4.5 to 5.0a with no trouble.  I also installed Sony Picture Motion Browser with no problem.  I also went into that directory and wrote some files to it. Changed those file names and back again to confirm I had full control and privileges.