This is entirely written by WAW8 on VistaForums http://www.vistaforums.com/Forum/Topic13820-21-1.aspx
In Vista, every device driver you install is copied into the "driver store". This "store" is not somewhere you go to purchase drivers; instead, it’s an area on the drive where Vista squirrels away device drivers for safe keeping. This is done so that, if you need to reinstall that device later, Vista already has the driver available.
Problem is, you may want to remove this driver so Vista doesn’t use it anymore — especially in the situation where the driver doesn’t work, and after you remove the entry from Device Manager, upon reboot, before you can install a different driver, Vista automatically reinstalls the driver from the "store".
What you will need to do is locate the actual device "package" you need to remove and delete it from the driver store.
To locate the device "package", open an elevated command prompt window (enter"cmd" in the start area, then press ctrl-shift-enter) and enter "pnputil.exe -e". This will provide a list of all the third-party drivers installed.
Search the list for the driver version you want removed. It will be oem##.inf. (where ## is the actual two-digit number of the inf file)
To remove that driver, using the same elevated command window, enter "pnputil.exe -d oem##.inf" to be safe, find all the driver files and remove them all.
Then, you need to clean out the INF directory:
1) open %windir%\inf\setupapi.dev.log in Wordpad
2) search for ", this will be found on a DevDesc line
3) look a few lines up for the "inf:" line. It will say Openeded INF: and at the end of the line, you will see the actual name of the inf file.
4) remove that file from the INF directory.
5) look for ServiceBinary=c:\Windows\system32\DRIVERS containing a reference to the same inf file. That’s the driver file to delete.
6) continue this process, search for all the inf sequences and DRIVERS references, removing the files from the INF and DRIVERS directories as needed.
Also, be sure to remove the device from Device Manager before you reboot.