Connection problem via Linksys WPC11 and BEFW11S4

Could anyone help me with the following curious wireless connection problem.
My HP500 laptop (using Windows2000) works perfect with a Linksys wireless
network PC card WPC11 ver.3 and the Linksys router BEFW11S4 ver.2 hooked up
to an ADSL modem and a standard PC with XP.
When I use the same PC card and same software on the HP Pavilion ze4400
laptop (using Windows XP) of my girlfriend, both the signal strength and
link quality are a lot worse even when both laptops are put in exactly the
same location. Furthermore the connection gets dropped regularly for unclear
reasons also when the signal strength is still OK.
I would be grateful for any suggestion how to solve this.
Regards,
Rob Boddeke