The 2 most promanent exsamples of of internet issues I have seen, are
1. Bad ISP.
2. setings.
Settings can result in no conection, a virus which affects that will do the same, firwall or virus programs can block ports(ways that data accesses your computer), which is the same; The easiest step is to lower your security settings, that inclues any firewalls and virus software, peerguardian is a good exsample, as it is set to block HTTP sites by default. I never did like the idea of a reformat, if you suspect a virus, post all details, (programs shuting down, crashing, computer rebooting, mouse pointer going insane, explorer going insane, cpu maxing out, extra processes starting up on the spur, settigs changed, anything obvious).