browser block

os = win xp pro
browsers= firefox, google, ie
windows firewall
AVG 8.0 free installed

After a fresh boot, browsers surf internet for about 6 min. and then freeze.
Internet connection live and active because email works.

If I try to open a browser 6 min. after fresh boot, I get a connection time out error but email works OK.

Can some bright and shiny guy/gal help?

Thanks,

John