Slow file transfer between computesr over LAN

All,

I have the following:

HP Desktop, XP Pro, SP3, with Maxtor external HD
Dell Laptop, VISTA Business, SP2
Both 32 bit
Both running Norton Internet Security 2011, including firewall
Linksys WRT54GL router connected to DSL

The external HD is configured to share over the network. I use it to
move files between the twp computers, and to access the Internet. It
has been working fine for about 3 years.

Everything is still working, except that the speed of transfer, either
direction, between the laptop and the external HD has dropped to
nearly zero. Files that used to take a minute or so to transfer now
literally take hours, and depending on the software, may time out.

The problem is not so much the speed over the network, but that the
data is only sent in short bursts, with long, many seconds, delays
between. As the file progresses, the process gets worse. Typically,
it takes as long to send the last 10 % as the first 90%. In the past,
the data was sent in a continuous stream. The access between the
laptop and the Internet is still fine.

I am in a rural area, and have verified that there is no other WiFi to
interfere. Using my backup program, I have rolled both my computers
back to a time when everything was working. The problem still exists.
That seems to point to the router. I have flashed the most recent
firmware and reconfigured, but not help.

Google turns up lots of hits, but nothing that fits this. Any
suggestions on what to do next? For now, I am just swapping the USB
drive physically.