[Nexus 7 Q&A, Help & Troubleshooting] bftb0:
Well, if you can't communicate with the device successfully, there's no hope of doing something correctly from the PC. So, try a few simple things on that angle of the problem first. The problem you are seeing could come from any number of places:

- damaged USB (or other) hardware on tablet

- bad / cheap USB cable

- damaged USB port on PC

- driver install issues

The symptoms you describe for fastboot sound a bit like the custom recovery got damaged or erased, although more severe (and not fixable) problems are also a possibility.

Since you previously installed a ROM, I would take it to mean that at one time you had at least one driver working (fastboot at a minimum), and possibly also ADB mode (in the OS and/or in the recovery). That's why I put "driver install issues" last in the short list above. If you never touched any of the drivers after you did your initial ROM installs, I would not expect driver issues ... but I don't know what you might have tried subsequently with the toolkit (which does fool around with drivers).

Are you using the OEM cable? Some aftermarket/charger cables have a short mini-USB connector, and one dollar cables off of eBay can be poorly constructed (bad shielding or impedance control, so they cause data errors at high speeds).

If you have an 2nd cable, I would try to see if you experience a different set of symptoms with that; same goes for trying a different USB port on your PC.

If you see errors in the Device Manager associated with fastboot I certainly would not be attempting any operation from the PC until you get that resolved. Note there is a huge difference between "unknown device" and a device showing errors - the latter suggests that the OS is aware of a driver which is supposed to be loaded when the tablet shows up on the USB bus, but then some malfunction occurs. That malfunction could either be hardware or an inappropriate driver.

Drivers

http://www.asus.com/Tablet_Mobile/Nexus_7#support_Download

http://xdaforums.com/showthread.php?t=1426502

Note that you should be able to use the Asus driver for every mode of your Nexus 7 (except MTP and PTP, which are handled by Windows generic class drivers), including your custom recovery by doing the following:

A) Download the Asus Driver and unzip it someplace.

B) make a backup copy of the "android_winusb.inf" file (for instance "android_winusb.inf.backup")

C) Open up the "android_winusb.inf" file with the Notepad program (Note: not Wordpad!)

D) Find the ";Google Nexus 7" stanza in the [Google.NTx86] section and add this line:

%CustRecoveryADBAdbInterface% = USB_Install, USB\VID_18D1&PID_D001&REV_9999

E) Add the same line (step D above) in the [Google.NTamd64] section

F) At the very bottom of the file in the [Strings] section, add the line

CustRecoveryADBAdbInterface = "Nexus 7 Custom Recovery ADB Interface"

G) Save the file and then proceed to do a manual de-installation of the drivers for fastboot and ADB.

H) Disconnect from the internet and then re-install the Asus driver for fastboot and ADB using the "Choose Location" method.

Don't know if that will work, but it's something to try; the problem you are having might lie outside the scope of what all of the above deals with

good luck