Sky Broadband Connects for a few seconds and then...

Question

Hi.
My girlfriend has SKy Broadband.She was advised to remove all antivirus which she has done.I can use my laptop wirelessly off the same sky box without any problem and when I've called Sky in the past they tell me it is connected their end.They have sent numerous Sky boxes and still the problem persists.When on the rare occassions it does connect, it only stays connected for a miute or so before cutting off.

Any ideas?I'm not particularly computer savvy so if you can make the explanation as simple as possible I would greatly appreciate it.
Regards,

Tony