What are the advertised speeds for upload from your ISP?
 
If you do not know then here is a TEST.

My problem seems to be inside the LAN. I have a LAN consisting of the Server 2003, and five Windows XP machines, and the LAN connects to the internet via a Netgear router. The router is the DHCP server and gateway, the five workstations are DHCP clients, and the server is statically assigned.
 
As for my speeds, using that test I get 4770kbps download and 355kbps upload. The ironic thing is I can only transfer to my server shares(internally, over the LAN) at about 40 or 50 kbps.
 

If you are getting 40 to 50 upload from those numbers you gave then that is right.
 
Look at the numbers below the graphics on that test as you will see more.
 
Example from mine:
 
Download Speed: 7055 kbps (881.9 KB/sec transfer rate)
Upload Speed: 353 kbps (44.1 KB/sec transfer rate)
 
As you can see i get 353 kbps upload which equals 44 KB.

Yes, however I'm not trying to upload to the internet. I'm am just trying to copy a file from one of the Windows XP clients to a share on the 2003 server, accross the 100mbps full-duplex LAN. The file never touches the router, and shouldn't be bottle-necked by the internet connection in any way.

I am also currently experiencing slow uploads and fast downloads from a Window 2003 server. All file transfers are taking place over the LAN and not via the internet.

I am having the same problem--slow upload to the server (2k server, not 2003, but still) but decent download, and only withint the LAN. Was any resolution to this problem reached? It merits mentioning that due to factors beyond my control, the antivirus software has not been updated since August '05, but i'm not aware of a virus that does this (not to say one doesn't exist).
 
Thanks for anyone who may have a solution or diagnosis.

A lot of times there is a problem with Firewalls. In any case, look at this article to see if it helps: http://www.smallbizserver.net/Default.aspx?tabid=139