If your Nook is running version 1.4 of the software, you may find yourself in my situation... Every time WiFi is enabled, it will download an update, and install it, with a five minute reboot... Once it has rebooted it will download the update again, and so-on until you turn off WiFi (by enabling Airplane Mode).
"Sideloading" - that is downloading the software update from bn.com, and copying it to the Nook using the USB cable doesn't help - the Nook sees the software update and tries to install it - but fails, and you're still stuck at software version 1.4.
It took me a Sunday afternoon and two very long calls to support to get it fixed... The procedure to fix it is as follows:
1. Power up the Nook - un-register the device.
2. Hold down the power button for 20 seconds.
3. Release the power button and hold it down for 20 seconds again.
4. Repeat the procedure 6 times total.
5. After the sixth time, let the Nook re-boot. It will announce that it is restoring itself to the factory shipped condition. - it takes quite a while to do this.
6. Re-enable WiFi, and re-register the Nook.
7. Sideload the 1.5 software - it takes a lot longer for this successful update to complete...