I'm also having installation problems, but I think it's related to Vista64 OS.
I started a thread in the Inventor sub-forum (
clicky).
If you're running Inventor '09 on a Vista64 machine, please post over there and tell me how you did it.
The long download completes, the installer starts, then immediately says something about how it "doesn't support this OS".
FYI: I'm trying this on an HP Pavilion dv5 Laptop with Vista64.
Thanks.