Windows 2000 - Network Configuration

on your router, you can assign a higher priority for different ports...

For example, you can set port 80 - the web - to a higher priority than other ports

You did not explain completely what is going over the DS1 (T1), are voice lines sharing it or is it only used for data ? Can channelize a DS1 to use 23 voice and one data, so you may have too small a pipe for data due to most of it being used for voice.

Another problem can be checked by running a tracert to a remote IP (home office) if extreme latency < 100 MS may have a TSU/CSU problem. IF you have a good TSU/CSU will have frontal user interface which allows you to see active/historical problems/errors on the DS1 or Router interface. If you have the DS1 plugged directly into the router may have a router CSU problem, check the logs on the interface for LOS/LOF/ES/CRC/BER errors (Loss of Signal, Loss of Frame, Escape from Sync, Cyclic Redundancy Check, Bit Error Rate exceeded), these indicate a problem with the circuit/configuration/CSU interface.

If this is confusing, call your DS1 provider and have them test the lines up to your equipment (CSU). A good provider will tell you if there is a problem with the line or your equipment, and never have to visit your office !

FYI - 1.544 Meg / 20 users = 77.2 KBPS or 1.38 times faster than dialup. File size and number of users on the server will slow it down even more.

I think you need a DNS server... this way every packet will be checked and resolve internally. The way I am looking up is that if there is a packet to be sent it is using a broadcast and the DNS at the main office is the one resolving on where to forward the packet.
You will reduce a lot of traffic in the T1 line if you have a Domain Contoller, DNS, and DHCP relay server at the branch office. Authentication alone takes time...
Is the internet also goes to the T1 line and the ISP is connected to the main office ? If yes try to limit the user from accessing the streaming audio/video and the internet screen saver.

Server now are not that expensive... With the way you mentioned it is advisable to have a server for if you equate the lost production time versus the cost of the server the capital investment could be recovered in a year time span.

Your best bet would be to look into a Citrix solution or at the very least a Windows 2000 Terminal Server. If you have a 2000 server at the main office that is doing much IE file server with plenty of memory (20users x 64MB/user)+ 1-2 GB for the server itself. Install Terminal services (application mode) and install the license svr on it. IF you 20 users have 2000 pro or XP pro they have the TS client access rights to the 2000 server (svr 2003 will require cals for the 2000 clients) Install the applications on the TS svr using add remove program. Try the performance of this config to gain better performance you can make the server a TS only svr (recommended anyway) and for further performance gain step up to Citrix on the TS svr.
Without goin the TS route you need to do some router config so that only necessary traffic goes accross the T1. (with Cisco you can even spoof the WINS)