We have 9 installed on a number of machines with Win7 pro 64Bit and no problems.
What we however did was remove the previous installs before proceeding. Generally we just keep the data folder if needed but got rid of everything else. You can just backup the notes.ini , id files and any other file you may think you need for 9.
I do recall my one developer installing over 8.5 and having an issue, however after it been removed, no problems.
Generally no issues. Hope you get sorted.