Neighbour turns up their microwave every fifth day, which interferes with your WiFi network. Nah, they'd have to live according to some calender that has five-day weeks for this to be anywhere near plausible.
ISP resets your connection every five days, or maybe you told your router to relinquish a DHCP lease every fifth day. Is that a router setting, or does one do that in the connection settings of ones pc? I dunno.
Can you ping an outside server when your browsers cannot go online? Open a command prompt (Start > Execute > cmd), then "ping google.ca", without quotes. This should show you if the connection is still alive. If so, something blocks your browsers. If no, it's time to troubleshoot your network.
Only if the previous test was positive; if you still have Opera installed, can you get online with it when Chrome and Firefox are blocked? This tells you whether the blockade is is specific or not.
Does the event viewer (right click My Computer > Manage > Logs, I have a Dutch version of XP so labels are a guess) show anything special?
Then there are some IP commands you could try at a command prompt: ipconfig /?, ipconfig /release, ipconfig /renew, there are not too many options.
Best regards,