If you run into drive mapping issues with drives previously mapped to Q or R Microsoft in their infinite wisdom has decided to use these drive letters for their own purpose. I had a client with a Q drive mapping that was confiscated by this click to run MS office issue. Once MS hijacks the drive letter it become protected...what a pita...here's the link for more info.
http://office.microsoft.com/en-us/products/what-is-click-to-run-HA101868855.aspx