There are times that it would be nice to mount a drive in Windows and set it to read only to avoid accidentally over writing data on that drive. After some Google searching this is what I found. This will only work on Windows XP sp2 and later and Windows 7. I have not tested it on Vista although I assume that it will work.
Start > Run Type regedit.exe
Navigate to the following registry key. It may not exist there for you will have to create it.
Next create a new DWORD called WriteProtect and give it a value of 1
Now plug in your USB drive and you will not be able to write to the device.
To make it writable again you will have to change the WriteProtect DWORD to 0 and unplug and replug in your device.