I was recently hired by a company to do some computer/network
technician work. They have been having problems with their T1 line so
they think. There is only about 15-20 users on the network at the
same time. The main programs being used are e-mail(groupwise), web
based scheduling software and a phone system program. A good portion
of the computers seem to be running slow a lot. I have run bandwidth
tests from PCPitstop and I get anywhere from 1000Kb/s(newer computers)
to 750Kb/s(older computers PII and PIII's with 128RAM). Some (old and
newer computers)also experience problems in their web based scheduling
software and get pages that send back an error that the page couldn't
be loaded or something to that matter (It wasn't the 404 Page Not
Found error.). We also have a Cisco PIX Firewall, two 3Com Switches,
Citrix Server and a Novell Server on the network.

Could the source of the problems be the Older PC's just can't handle
what they are doing and can't process stuff at the speeds of the T1?
Could it be the PIX, Switches or Servers (or their configurations)?
Or could it be the T1 line? Or could it be in the Network Cabling?

Any help would be great.