Skip the "file:" protocol (if any) at the beginning of the path name in TWinNTSystem::ExpandPathName

This fixes the many errors as the one shown below, when running $ROOTSYS/test/stressGeometry:
Info in <TGeoManager::Import>: Reading geometry from file: http://root.cern.ch/files/aleph.root
SysError in <TFile::TFile>: file file;./files/aleph.root can not be opened (No such file or directory)
Error in <TFile::Cp>: cannot open destination file ./files/aleph.root
Warning in <TFile::OpenFromCache>: you want to read through a cache, but I cannot make a cache copy of ./files - CACHEREAD disabled