Internet Connection Sharing

I am unable to share an internet connection between two computers in a homegroup. File sharing is OK. The ADSL modem is connected (via USB, no ethernet) to a desktop, the desktop to a laptop via a router.

The problem is that after opening network/adapter settings/LAN/properties on the desktop, NO sharing tab appears, only a 'networking' tab. On the laptop both tabs appear...

How can I get around this? I know I should really get a modem that connects directly to the route, but don't want to waste any time/money if it doesn't resolve the problem.

Thanks

From that site -

Note

The Sharing tab will not be available if you have only one network connection.


This must be the crux of the problem, I have one network and one internet connection. Obviously i want to share, why no tab?

Btw, thanks for your reply Carolyn, but in fact network & sharing says that I have 'multiple networks'.

Yes it is set to automatic.
I've installed a new modem/router and now have internet access on both computers...still no 'sharing' tab though. I used to have internet connections between windows 7 and XP, then windows 7/windows 7 via a switcher and a couple of ethernet cables, but it would regularly not co-operate for unclear reasons (unidentified network appearing at random, for example), so I don't know how long this set-up will last.