I have a better understanding of my problem :
- on Windows XP I detect 3 WiFi routers in my neighborhood ;
- with Ubuntu Karmic :
. wicd detects 4 WiFi routers (3 on Channel 11 and 1 on channel 1)
. wifi-radar detects 5 routers (3 on ch11 and 2 on ch1).
As I am set on channel 11 I'm facing interference from 2 other routers and
according to WiFi documentation I could read there should be a 5 digit
separation between two closely used channels to avoid interference. I should try
to set my router on 6 and will look more deeply in the documentation.
Sorry for this long discussion, in which Ubuntu has probably nothing to do,
being probably more sensitive than Windows XP facing interferences.
Thanks for the answers and help I got.