If you're comfortable using the command prompt, you can use the DOS command: subst. A better and easier way to create the drive association is using a small program called Visual Subst. It's basically a GUI front-end to the subst command.
Here's how to use Visual Subst to associate a drive letter to your Google Drive account:
Step 1: Download and install Visual Subst.
Step 2: Select the drive letter you want to associate, then type in the path of your Google Drive account, or browse to the path. In Windows 7/Vista, it should be C:\Users\Username\Google Drive, where Username is your Windows username.
Step 4: After the association has been added, the virtual drive will appear in the Visual Subst list.