SBS 2003 slow down suddenly

I have a fresh installed SBS 2003, SQL server, 2 Nic, Line 50 - 10
Users, TREND SMB V3, all done 1 week ago.

I migrated from an SBS 2000 box using the Microsoft White paper and the
ADMT. All went very smoothly, accounts moved correctly etc.

Now I keep hitting a problem that the network goes incredibly slow for
what is no reason I can see. Rebooting the server sorts the problem for
a few days (2/3 if lucky) then starts again.

On the client side I have XP SP1 and SP2 machines and during the wobble
I can ping the server by IP and by FQDN to test my DNS out. It gets so
bad that even the start button willl not pop up on the computer whilst
using it, email goes out the window and browsing of shares etc too.

The logs dont show DNS errors, or any network issues ? Im worried its
my SQL database as that grew so hefty before its the reason they moved
on to new server. Perfomance counter shows a bit of disk usage at the
times but nothing else.

What should I start to log ? SQL or DNS or DISK, It does seem to be
an afternoon thing around 12 / 1pm but have turned off the shadow copy
sched task. to see ?

.