Windows – How to get list of all network drives mapped to the system

For this kind of problem, USB Drive Letter Management (USBDLM) makes wonders. You can allocate drive letters to USB drives, make sure it does not interfere with network drives, ...

From their Getting Started section:

When a removable drive (USB flash drive, flash card reader, portable hard drive) is attached for the first time, Windows mounts it to the first available 'local' drive letter. If there is a network share on this letter, Windows XP will use it anyway for the new USB drive because since XP network shares are specific to the current user and not visible in the context of the system where the letter is assigned. The USB drive then appears to be invisible. This is fixed by SP3 in most situations. You can change the letter assignments in the Windows Disk Management Console with a lot of mouse clicks but you have to do it again for every new device. And, for USB devices that have no serial number (in violation of the USB standards) you have to do it too when you attach it to a different USB port. USBDLM can for newly attached USB drives

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check if the letter is used by a network share of the currently logged on user and assign the next letter that is really available

This is a bad idea. The problem is mapped drives can vary per user, and so you don't know what mapped drive E: means until after you've already logged in and loaded your profile. You might get it working via UNC path (\\server\share), but I still don't recommend this for the whole profile.

A better alternative is either using Roaming Profiles (but IIRC that requires Active Directory) or mapping only specific folders (Documents, Desktop, etc).