In article Hide drives from the Send To menu in Windows XP, we saw how to hide drive-letters from the Send To menu and in Explorer. This article describes how to accomplish the same under Windows Vista.
By enabling the NoDrivesInSendToMenu policy, you can hide drive-letters from the Send To menu in Windows Vista without actually hiding the drives in Windows Explorer environment. As indicated in this article, NoDrivesInSendToMenu is not applicable for Windows XP systems.
HKEY_CURRENT_USER \ Software \ Microsoft \ Windows \ CurrentVersion \ Policies \ Explorer
If the above branch does not exist by default, you'll need to create it.

Fig 1: Drive-letters displayed in the Send To menu

Fig 2: Drive-letters hidden in the Send To menu