Browsers won't work but XBL will

Hey guys, throughout my household we have about 4 computers. It's becoming a huge pain that occasionally my mother and sister complain about their browsers not working and the internet has to be rebooted. This works, the internet is rebooted and browsers are restored again.

My problem is that I'm quite often either playing with mates on Xbox Live or playing games on the PC when it happens and my Steam, Xfire and games in general all work well. Most of the time my browsers work aswell.

We've established it's not the games causing it because it also happens when I'm not there. My sister uses XP and my mum uses Vista. Any tips would be appreciated because the more we restart the modem the more the problem appears to occur.

Also I've tried most of the fixes from the previous "My Browsers dont work" topics and none work.