With ISA Server 2006

Dears
i have an ISA server 2006 with a dsl internet connection , but the point that i couldnt access any web site from ISA without adding a proxy address (from my ISP)
although i can surfing the internet without proxy on other machines……by using just gateway
on ISA sever i use the same gateway, and an access role to allow every thing
but i still have that problem
any help!!!!