Intermittent network delays

I have a Win 2000 Server network with 19 Windows xp
clients in a building with 10/100 ethernet connections.
The computers have been recently upograded (oct-nov) from
various Windows 98SE, ME platforms to XP.

For the most part, everything runs fairly smooth, with
the exception of seemingly intermittent 30-45 second
delays when accessing files on the server. I might be
able to access several files in a row and get instant
network response (these files are cad so 1/2 - 2 MB files
are not unheard of) then I'll open another file and I
have to wait nearly 15-20 seconds for it to begin
transfering the file. The same sort of thing happens
with Word and Excell files.

On one machine, (I am convinced it is related), it takes
nearly 60 seconds to open Outlook 2000(which accesses
Microsoft Exchange) 2-3 other machines experience this
delay also but not for 60 seconds.

I've searched the newsgroup but thought I'd get faster
response posting. I don't think this NETBeui is the
answer for me although I remember it being the default
protocol at one time. (I guess NETBios took over). I
did set the netbios over TCP/IP and that helped cut out
some of it but not all of the delays.

I'm almost leaning toward looking at a Larger packet
protocol or something to do with large single files. My
firms computer techies tell me I need to roll back to Win
98 or roll back to Outlook 95. I'm willing to live with
the delay before I roll back.