The most likely cause is a dodgy USB cable - try a different one (you can get them for around $1 on ebay).
Failing that, it's probably a driver issue, in which case you need to:
- disconnect the nook from the PC
- uninstall all related drivers and software from the PC
- restart the PC
- restart the nook
- reconnect the nook
NOTE: do NOT remove the nook until all the drivers are installed