I really think this is one of those cases where DOS is your friend. Go to the RUN command and type cmd to open a command prompt. The first command to type is "ipconfig" to see if you are getting an IPAddress that is a good start. Your address should be something like 192.168.1.100. If your address starts with 169. then you are not getting a valid address from your router. If you are getting a valid address you can extend that command and type "ipconfig /all" and look and see if you are getting a valid DNS. You can also check DNS by pinging a name like "ping google.com" you should see something like:
C:\>ping google.com
Pinging google.com [74.125.227.105] with 32 bytes of data:
Reply from 74.125.227.105: bytes=32 time=231ms TTL=53
Reply from 74.125.227.105: bytes=32 time=29ms TTL=53
Reply from 74.125.227.105: bytes=32 time=29ms TTL=53
Reply from 74.125.227.105: bytes=32 time=28ms TTL=53
Ping statistics for 74.125.227.105:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 28ms, Maximum = 231ms, Average = 79ms
If Google is not found you most likely have a DNS issue.
you can further test to see if it is connectivity or DNS by pinging google's address which looks this:
C:\>ping 74.125.227.105
Pinging 74.125.227.105 with 32 bytes of data:
Reply from 74.125.227.105: bytes=32 time=50ms TTL=53
Reply from 74.125.227.105: bytes=32 time=29ms TTL=53
Reply from 74.125.227.105: bytes=32 time=29ms TTL=53
Reply from 74.125.227.105: bytes=32 time=29ms TTL=53
Ping statistics for 74.125.227.105:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 29ms, Maximum = 50ms, Average = 34ms
If all these tests succeed than you may have your connectivity set up wrong in your Internet Options.
You can check this by opening Internet Explorer and going to "Tools" then "Internet Options" (You can also find "Internet Options" in the Control Panel. Once you have it open look for the tab at the top that says "Connections". For most people it should say "Never Dial a Connection". The only exception to this would be if your ISP (Internet Service Provider) did not provide a router, but supplied just a modem. If this is the case I would suggest you invest in your own router, I have bought wireless routers for as little as $15 (US).
I doubt changing the browser will fix this because if it was the browser it would not normally work sometimes and then sometimes not.
If you are still having an issue please give more information like what kind of broadband you have (e.g. cable, dsl, dialup). What kind of Anti-Virus programs are you using. What kind of Firewall on the PC. This could be just Windows or your Anti-Virus program could have also installed a Firewall.