[plt-scheme] can't connect to PLaneT on Windows XP machine

I'm trying to get PLT Scheme to work on a Windows XP machine, and I
can't add needed modules from PLaneT. When I do so in the customary
way, I get the following error:

---
require: PLaneT could not download the requested package: tcp-connect:
connection to planet.plt-scheme.org, port 80 failed (at step 4: An
operation was attempted on something that is not a socket.; errno=
10038) in: (planet lizorkin/ssax:2:0/ssax)
---

I've looked both on the web and on this mailing list and didn't find
anything useful. I did however learn from one page to check to see
whether port 80 is being used by other software. I checked, using the
Windows command-line utility netstat, which showed no sign of any
"active connection" using port 80.

I also consulted Internet web pages telling me how to open port 80, as
well as checking my antivirus software for possible conflicts; neither
of these helped.

Anybody got any ideas? Thanks.