Right click on Bluetooth USB Host controller and choose Properties > Driver > Update Driver. Browse my computer for software > Browse > navigate to where the installation files are see below
C:\dell\Drivers\WLAN & BT 4.0\BT_Device_Driver\64
Click Next and see if it installs the driver.
If that fails then uninstall the WLAN + BT Client Installation from Control Panel > Add/Remove programs and reinstall the drivers again
If it still fails my guess is that you didn't install your drivers in the correct order when you installed the new OS
solved, formatted & re installed win 7 with dell drivers, working fine but a minor glitch, my BB 9790 cannot be paired, all other are compatible except my BB, still to try in win 8