I pay for Google Apps for business for my side company 10x13. I mainly just use it for email. I don't really use it for document or document sharing. But I did notice today that I had a 30 GiB drive with my paid for account and I thought I should put that to some good use. So I decided to figure out how to mount it as a drive, so I can do a few simple off site backups.
Google's install guides for Windows and Mac can be found here https://support.google.com/drive/answer/2375078?hl=en 
Mount as a drive in Win 7
Click on Download Google Drive for PC
Run the installer
It will download the actual installer
Click on Sign in now
Enter your username and password for your Google account and click "Sign in"
Click "Start Sync"
After the sync it opened up a folder window that is my synced Google Drive.
It also added a Google Drive to my favorites.
Out of the box it creates and syncs to a folder at C:\Users\patman\Google Drive
This is nice, but it's just syncing a folder. I would rather mount it like as a drive.
One simple way is the use the SUBST command.
Open a command prompt and run the following command.
> SUBST x: "C:\Users\patman\Google Drive"
Give it a few seconds and….
An X: drive is added (which is from the C: drive so it will have the identical amount of free space)
The bad news about this is it won't survive a reboot.
The good news is there is a little program that will fix this. Psubst which can be downloaded from https://code.google.com/p/psubst/ 
Go to the download page https://code.google.com/p/psubst/downloads/list.
Open the zip file and you will see the psubst.bat
I put the psubst.bat file in C:\Windows\System32 which is on my Path variable.
Then from the command line run the following to first unmounts the x: drive.
> SUBST x: /d
Now use the psubst tool
> psubst x: "C:\Users\patman\Google Drive"
That worked, but after a reboot I got this….
So it did not work…. They recommend setting up a registry key to handle this.
As an alternative I found this http://howto.cnet.com/8301-11310_39-57426705-285/how-to-map-a-drive-letter-to-google-drive-in-windows/ 
It can be downloaded from here http://www.ntwind.com/software/utilities/visual-subst.html
Click on Install
Click on Close
Open "Visual Subst"
Choose a drive then click on the search.
Click on the '+' button.
Select "Apply the virtual drives on windows startup and click close.
Click on Close.
The drive now shows up
Now reboot to see if it stays.
And after a reboot
It is still here yahoo!
 Install Google Drive on your computer
 Google Drive Site
 How to map a drive letter to Google Drive in Windows