Im having a similar problem. I can start inventor fine and everything works until i click a tool like box or line then it sezies up the entire computer. I am running windows 7 x64 with an I7 processor and 8 gigs of ram. It was running fine as of a few weeks ago.
You said the other 2 computers you have capable of running Inventor run Vista? Have you tried installing it on one of the others to see if the problem persists? This could possibly eliminate Vista as the cause.
Can you upgrade the RAM? Most times it is relatively inexpensive to upgrade to 8GB or 16GB if it will handle that much (age of computer). What type of video cards do they have or worse yet do they have integrated video drivers? Is your Vista computer with 8GB new enough to handle Windows 7? I think an upgrade to Windows 7 from Vista is around $120 bucks and should tell you if your computer can handle it. Start Menu>Windows Anytime Upgrade.
Install it on the Windows 7 computer. Although with 1GB it would crawl, it eliminates windows as the problem if it works. If it doesn't it may be video drivers or your video card may not be robust enough.
When you installed IV on the machine, were you?
1.) Logged in with Administrative Privileges
2.) UAC slider set to lowest position (off)
3.) Anti-Virus (if any) disable and it's service stopped
The first two are prerequisites, which I guess would make them important.
Un-install Inventor and re-install with the required settings.
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