I am assuming you have norton anti virus installed. It sounds like you may have a virus or worm. Go to this site and download the free trial version to your desk top. Then unplug your modem and uninstall your anti virus, next install the F-Secure then plug your modem back in and up date F-secure and run a scan. The reason for doing it this way is the first thing a virus or worm does is disable your antivirus.
If everything comes up clean you can uninstall f-secure if you want to and reinstall your old antivirus. just make sure you are not on the internet though. Do the same for your sister's pc
Do not try to run two antivirus programs at the same time all sort of problems can happen.
Do you have any antispyware installed? If not I will give you some links for free stuff.
http://www.f-secure.com/download-purchase/list.shtml