Windows 7 has a flaw : it can not deal more than 8 USB-Midi drivers. So at some point, you may install drivers and never see your device plugged-in.
The trick is to manually delete some registry keys in order to free these slots. Usually you don't deal with more than 8 device at the same time. It's usually old drivers you don't use anymore. In the worst case, you delete a usefull driver (your sound card) and you will have to download the latest version in the internet, which is not a bad thing as well.
This Video gives details but you don't need a 15 minutes explanation since you just have to :
- launch regedit
- reach HKEY_LOCAL_MACHINE/SOFTWARE/Microsoft/Windows NT/CurrentVersion/Drivers32/
- Here you can see keys called "midi1", "midi2" ... up to 10 and more .. Delete the latest, until you only have 4 (for example .. You can also delete all these keys and reinstall all your midi stuff to be sure)
- Reinstall your Korg or other device : it should work now !