internet connectivity

on a clean formatted internal sata HDD I installed Vista ultimate and started to config an internet setup using my ISP's instructions. when connected it is OK ,my virus def's are downloaded and windows updates are downloading___on access to my status it shows that I have in internet connection from this computer to home which is my ISP and a connection from my ISP to the internet (dialup) everything is working fine untill I start my browser and mail, in fact any internet activity will disconnect the internet back to my ISP,the connection to my ISP is still active______this has been ongoing for 1 month,and I have tried many options, My computer is running 4 HDD's plus a rack caddie which is never connected for weeks, the HDD that I am using now has XP Pro installed,vista is installed on another HDD____help is needed here with this one--just ask for any details and error messages__Bill in OZ