The Nano 2nd Gen is still "Unsupported" ie. it still has some kinks to be worked out.
...but I used to have this problem with my 2nd Gens as well.
Try setting the USB HID to the opposite of whatever it's set to now.
Settings/General Settings/USB HID
Also, my PC seems to hate my 2nd Gens when the USB Keypad Mode is set to anything other than "Multimedia" (and it ESPECIALLY hates the Mouse setting), but with the 1st Gens....it's all 100% fine.
(I haven't checked if the 2nd Gen still does that with current SVN in a while now...I just don't use it)
Settings/General Settings/USB Keypad Mode
Let me know if this helps,
[St.]
PS: As a Nano user....could you (if you haven't already) take a look at:
http://themes.rockbox.org/index.php?themeid=406&target=ipodnano2gMy latest Theme for the Nano
