Access Denied to Internet

hi all.

Recently my browser (Firefox/IE) keeps on saying me that you are Access Denied to <xxx> page, please contact cache admin. im connecting to internet via proxy, therefore probably the cache admin would be the administrator of the proxy admin, but im sure that there is nothing to do with proxy because my other peers are having access to internet via the same proxy withiout any problem. Therefore im nearly sure this is because of some virus/spyware.

As soon as i log on to computer, i would have access to internet, bu after awhile im prevented to access any wed page saying access denied. And if i keep on refresh page i rarely got a chance to load the pages. Im current using kasperky anti-virus 2009, but it doesnt prompt me about any threats. And I noted that only web page accessing is denied by this virus/spyware while allowing to continue with background downloads.

can somebody please help me to get out of this problem, otherwise i will have to reformat my entire hard disk .

thanx in advance
shash