Server = Microsoft Small Business Server 2003.
Clients = Windows XP SP2 with all updates.

Problem:
From time to time, it takes Excel 60 to 120 seconds to open a file on the
server.
When this happens, all other users will experience this delay, i.e. in the
mean time they can't open a file.
It looks that the server has to work on the first request,
and once this is cleared, it can process to the next one.

Symptoms:
- On opening files on the local computer, there will never be a delay.
- When you are the only one working in Excel in the network, no problems.
- The problem starts, when 2 or more clients are working with Excel.
- We have Symantec Antivirus Corporate Edition 10.1.4 with patches.
Turning on or off the Antivirus, either on the client or on the server, or
both, makes no difference.
- Clearing XLStart on a client has no effect.
- Disabling digital signature on lanmanserver and/or lanmanworkstation on
server has no effect.

We only experience this problem since 1 or 2 months.

It's an extremely annoying situation.
Thanks in advance to who could solve this situation.