Example: i usually disconnect her cable when she's working (naughty mr) so i can get the highest internet speed ... when i plug the cable back in my connection works for some time (an hour, maximun 2), then it goes down to zero ... after that unable to access internet unless if i unplug my sister's cable ... i never had this problem with my old PC (i was using XP then ... does it matter ... i guess not cause my sister also has Vista on her computer)
I'm wondering if your sister has something slurping up the bandwidth that she isn't aware of. Maybe ask her if you could clean her computer up for her w/ a good spyware , antivirus, and then install a good firewall as well.