I would say that while installing 9i in windows 7, windows 7 itself warns you that this software has known compatibility issues. I visited the compatibility center and saw all clean, yet windows 7 warns me the same way. I tried doing that for the same purpose you mentioned in your question, but all efforts were futile, since some of the packages do not install properly, and O9i fails to function as desired.
options: use xp mode, or else use a VM within windows 7, and run windows xp in this vm and install o9i there. would seem like too much, but this is the best way to go.
In case it is just queries you wish to run, and not use any oracle specific features, then MySQL is a good option, especially the one with the GUI.