Thanks to you both for your advice. The situation so far:
iwconfig said - lo - no wireless extensions, ra0 - Ralink STA.
The Wicd preferences page, for the WPA Supplicant Driver, offers these options - wext, hostap, madwifi, atmel, ndiswrapper, ipw and ralink legacy. I selected the ralink legacy option.
I changed the Wireless Interface setting to ra0. On the External Programs tab I've got everything set to automatic.
After these settings were changed as above, the laptop now finds the availalble wireless networks, but I can't connect to any of them. I can't connect to my own router, nor could I get it to connect to my friend's network, for whom this is being done. When I try to connect it says - 'Resetting IP address' then 'Obtaining IP address' - then there's a long wait until it says 'Not connected'. It does this with my own Belkin router and my friend's BT router. I've checked the passwords and protocols as well as I'm able. I've also tried various combinations of security protocols, just to see, but still haven't been successful. So I'm stuck again.
I also downloaded the VL7 live CD, as suggested, which boots ok and the progress bar goes all the way to the right, but then it just cycles round and round between a black screen and a few seconds of showing the progress bar all the way across. My thoughts are that this very old laptop, with only 128MB RAM (which I've been unable to find compatible memory upgrades), is happy running VL6 Light, but not VL7 live. Any chance of a VL7 Light CD soon? So i was unable to see if VL7 will cooperate with the Tenda W311U.
Any further help or advice would be gratefully received. I'm hopeful htis problem can be cracked, it's just finding the way.