We have a handful of Windows XP SP3 users whose mapped drives don't always appear in Windows Explorer when they initially log in. "net use" shows the drives are mapped correctly and they can see them in File Open dialogs in other applications. However they don't appear in Windows Explorer.
The current workaround we have is to kill Windows Explorer and then immediately start it again, then they do reliably appear.
UPDATE: Still seeing this problem on some of our PCs. We're in the habit now of checking for any foreign DLLs being loaded into explorer.exe (Novell, Roxio, etc) and removing them, and making sure the PCs/users are in the correct OUs. This seems to solve the problem most of the time, but we've still got a number of PCs that won't show their network drives in My Computer when the PC first starts up even though they do show up in "net use" in a Command Prompt. But if you kill explorer.exe and launch it again, they do show up in My Computer. Anything else we should be checking for?