Mmm. Is it perhaps one of your Adobe products? Or AVG? One way to tell, is to remove all the non-Microsoft items from Startup, then reboot the system turning each item on, one at a time till you find the culprit - it is time-consuming though.
Another option is to check if you can spot it using TaskManager........
This might be better. use Control > Panel All Control Panel Items > Notification Area Icons and see if you can find it in there.