Notification area icons are sometimes invisible in Windows 7 (Boot Camp)

For some reason, sometimes my notification area icons are invisible in Windows 7 64-bit (Boot Camp, on an iMac mid-2010), as seen in this screenshot. Clicking on an invisible item does nothing. Restarting the computer will fix them, though. These icons are invisible during about 25% of all Windows 7 startups.

This installation of Windows 7 64-bit is completely fresh; I just installed it, then installed the latest Boot Camp drivers, and that's it.

Anyone know what's going on here and what I can do to fix it?