Try looking under
HKEY_CURRENT_USER\Network that will show you the current mapped network drives for the current user based on the drive letter. Or
HKEY_Users\<sid>\Network for a particular user (based on the SID).
While my answer will show what drives a user has mapped to a drive letter, adric's will show you the drives accessed that were not mapped to a drive letter (via UNC, etc).
Same path concept applies:
Specific User (by SID):