I cannot connect to the internet. Trojan problem?

I have a HSDPA device to connect to the internet. Just recently I had difficulty connecting to my yahoo mail and some other websites because of virus alerts that trojans have penetrated my system. I've removed them using AVG and now here I am starting to have difficulty in internet access. Other websites work perfectly fine it is the email and search engines which I've encountered problems. I've tried the HSDPA device with other laptops and they work fine.

Whenever I'm in a wifi hotspot the same problem occurs prompting me to conclude that there something wrong with my system.

Will reformatting my hard drives and OS re-installation do the trick?