I struggled with this error message for quite some time on one of my home computers.  When I tried to install some software like Adobe Reader or iTunes, the installation would fail and show me a popup alert saying "Invalid Drive: P:\".  This was really frustrating, because it was keeping me from installing some programs I really needed.  

I originally mapped the P: drive to the shared "Pictures" folder on my Windows Home Server (which is very cool by the way), and also changed the "My Pictures" folder for all users to point to that drive instead of the default location.  I did this so everyone would access and store pictures in the same place, and that would be on the Home Server which I had set up to have raid-like fault tolerance (can't loose those pictures).  Since that time I figured out a different way to do this, but although I had disconnected or "unmapped" the P: there was still a key deep in the registry pointing to this location for "My Pictures" ... and that was causing the conflict.

To resolve the issue, all I had to do was go into the Registry Editor (run "regedit") and navigate to folder shown below (the path is shown at the bottom of the window).

I just changed the value of the "My Pictures" key to point to the default location of "C:\Users\Cal\Pictures" ... and the next time I tried to install those programs everything went smoothly.