I had this unique problem on my parents laptop, certain sites such as facebook, yahoo groups, hotmail would be able to log in but thats about it, nothing else would work after that with the page just timing out when you click on anything.
Mail on the POP3 would download but SMTP mail send would fail occasionally by timing out.
Wondering why my ISP was preventing this from happening. The Airtel Beetel Modem is connected to a Netgear Router for wireless access.
It turns out that the issue was a combination of my ISP along with the wireless Router. There exists a setting on the wireless routers called MTU size in bytes. This typically is found on the WAN tab of your router and is set to 1500 bytes. The type of connection between the modem and the wireless router is a PPPoE type of connection. For this sort of connection the ideal value for this parameter is 1492. The technical details of this can be found on the following link.
By changing this parameters all the problems just disappeared ! Further read up has shown that this could cause problems to other services as well such as telnet / ftp / https / VPN like securepoint etc.
In order to change this setting, log into your wireless router and navigate to the WAN setup link. Make the change and click apply.
Do let me know if this has been helpful by dropping me a comment.