Find out if your PC is 32 or 64 bit and download the necessary driver:
32bits Windows xp,Windows vista,Windows 7: Download
64bits Windows vista,Windows 7: Download
Open up DS3 tool
Go to Driver Manager, select the driver and click "load driver"
Go to the profiles tab
Now select "Playstation 3" mode and click the pink "Enable" button
Select "Playstation 3" (Even if you're using an Xbox controller), and click on "Options"
Make Button 5 Digital L2
and Button 6 Digital R2
If your controller isn't working you may just need to wake it up by pressing the middle button. If nothing is showing up under "Connected game controllers" you might just need to restart your computer.