I have a friend visiting. His laptop is not 'seeing' my routers signal, nor
two other local signals that I can see on mine.
His picks up the signal from his own router when at home, so why doesn't it
see mine?

Brickcounter wrote:
All using the same standards ? A/B/G ?

http://en.wikipedia.org/wiki/IEEE_802.11

There are also some differences in channel allocations between countries.

http://en.wikipedia.org/wiki/List_of_WLAN_channels

I expect the software you are using, is at least
as good as something like this. I wouldn't expect
a different piece of application software to change
things, but if your current software won't present
everything it can see, maybe a tool like this might
make a difference in listing what is out there.

http://en.wikipedia.org/wiki/NetStumbler

It is possible that the README here, will help
identify other reasons it isn't working (SSID).

http://stumbler.net/readme/readme_0_4_0.html

Perhaps some wireless newsgroup, has a checklist
you can go through.

Paul