File access problems on server 2003

I have a 15-workstation network (mix of Win98 & XP pro running Office
2000 pro) that has had it's NT4 domain&file server replaced with 2003
domain&file server.

All machines have latest service pack & updates for OS and Office.
The server has an ADSL connection to the internet and Internet
Connection Sharing is active.

Since the new server was installed Excel will often hang when loading
spreadsheets from the server. Sometimes, if left alone for a few
minutes Excel springs back to life but usually it remains as "Not
responding".

On any one workstation I can load, say, 15 spreadsheets from the
server into Word without any problem but in Excel rarely get beyond
the second or third.

While Excel is hung I can load other programs (Wordpad etc.) but any
attempt to load Explorer will result it it hanging too.

If the spreadsheets are loaded over the network from another
workstation then there are no problems.

I've disabled SMB signing on the 2003 server in registry and Group
Policies.

Other applications (Sage Accounts & Payroll) access the server with no
problem at all.

All advice appreciated.

richard.