Poor Performance after upgrade

I have recently replaced our aging Windows NT Platform with a WIndows 2003 Server. All data and accounts were migrated from the old box to the new. I expected a much improved performance across the board but to my horror have discovered just the opposite. Server based applications that used to load in seconds now take minutes to load. Basic Word documents stored in users folder can take 5 minutes to open and regularly get NOT RESPONDING Messages.
Nothing else has changed so obviously, it is down th the 2003 setup. Has anyone got any ideas or seen this themselves?