[Hampshire] [?OT tech?] Weird update connection problem

I have been installing GNU/Linux on someone else's computer. I first
installed Lenny. I had no problem with the base install nor with the
automated installation of the standard system. But at the resulting command
prompt I got stuck. aptitude could not connect to any of three different
mirrors. Ping worked, both by IP and by URL, but Debian mirrors were a
no-no. The problem was the same for either static IP or DHCP.

So I gave up and reinstalled Lenny in case it was a faulty installation. When
it rapidly became obvious that the problems were the same, I started to try
other distros. I used the alternate installer for Ubuntu Karmic first. As
soon as I started to have problems, I switched to installing Kubuntu K from
the live CD. That gave me the GUI installer, so I have no idea what was
going on, but it crashed after 40 minutes with the error message that it had
crashed.

So I gave up on the *buntu stable. It and I have a checkered history. And I
installed PCLinuxOS 2009-2. (I was pretty desperate to have the computer at
least usable by the time the owner got home).

That was fine. Everything worked except.... connecting to the repositories.
I tried at least three different repositories and Synaptic, aptitude and
apt-get. I tried editing the repositories both automatically and manually.
Nada, zilch.

Whatever the problem is, it is clearly not the distros, since they all agree.
But what is the problem, and how do I solve it??

Could a firewall setting, if inadvertently set in the router, distinguish
between mirrors/repositories and everything else? If so, which setting
should I be looking at?? Perhaps reset to factory defaults and strengthen
the password? (It's amazing how asking someone else a question can clarify
one's thoughts!! But I can't get access to try now.)

Any ideas as to what I can try when I next have access, most gratefully
received. I have already decided to take another similar router with me in
case the router is faulty. But if it is faulty, how does it so accurately
distinguish whether something is a mirror/repository or not???

TIA
Lisi