This is usually happens because there is a malware/virus/trojan on that partition of your hard drive.
The first step is to make sure that the virus is deleted. Run a virus scanner of choice and make sure that it has the latest updates. Run a full scan and your likely to find a few files that are infected and your antivirus will delete them.
However, when you click on the drive letter in My Computer or Windows Explorer, you are prompted to chose a program to open the drive.
In order to fix this, download this little tool: Fix Drive
Run that tool and select the drive that has the problem and click 'Fix'. Now your problem should be solved.
If for some reason it is not, try the methods described HERE.