Office 2003 files slow over WAN

I have recently upgraded my users from Office97 to Office2003. My company has around a dozen remote offices ranging in size from
2-12 users each. All retrieve their files from a central server at another location.
The problem is since the upgrade, it is taking several minutes to open files and several minutes more to save those files. Does anyone have any suggestions on speeding up access?
Any advice would be greatly appreciated.