I have both 10 and 11 on at work. From memory 10 needs to be at least SP2 for 11 to install properly, then when switching back and forth you may get "Inventor needs to register with the operating system.." advisory.
I would post at Autodesks Inventor forum. We had a number of problems intalling R11 at work, had VAR in twice and AutoDesk helping via a citrix session twice. Turned out to be a network issue we finally resolved ourselves as well as some other documented issues you may or may not encounter. Stand alone install may go much smoother as I had minimal issues at home. Many of the lockups and other problems have disapated with the latest service pack on R11.