From what you have mentioned this could be some problem with your Internet Modem/Router OR it could be software malware even..
First up, can you try connecting another pc/laptop to the same modem and see if this "disconnection" problem occurs ? If it does not, you know you have isolated the cause of the problem with your dad's Win7 computer
If the second pc/laptop also seems to have the same problem, then you know its more likely to be a problem with your modem/router/lan
If its the 1st case, and a problem with your dad's Win7, then update your antivirus, update your anti-spyware software(Eg Spybot S&D) and run full-system scans for both viruses and spyware/malware, etc
If its the 2nd case, then you might need to get in touch with your InternetProvider to help you with diagnoising why you are facing this problem, since Router's also have a set of configuration and diagnostic settings+tools built within the router.... Besides your InternetServiceProvider CustomerCare might have seen these kinds of problems in the past, and so might be able to put you in the right track in getting this problem fixed..