We've got numerous Windows 7 machines that will show a mapped network drive as disconnected on boot up. Seems to be all versions of Windows 7 as well. In the "My Computer" windows the mapped drive is visible, but with a RED X on it. If you double click on it, the red X goes away and displays the contents of that share. I don't believe it's a timeout issue (as mentioned in http://support.microsoft.com/kb/297684) because this occurs imediately on startup.
SnapOverflow
This is a common issue with Windows 7, the best solution I've been able to find is to run a batch file that remaps the drive upon logging in.
It occurs because you're not connected to the mapped drive on boot up, under Windows 7 the drives only connect when you first access them.
Later on if you don't access them you can encounter the timeout issue.
All of our Win 7 machines exhibit this behaviour.
We have found that the more modern the hardware the more likely the problem. Often because the OS is up and running before the Network Card has fully initialised. We've had to generate scripts for pretty much all newer PC's.
On really fast PC's we've had to delay teh scripts by 30s to 60s to be certain the network card is fully active.