know it is not related to linux but it is related to networking

i have two dsl connections

one 512 and second 128

so now 512 is connected to linux box and squid is running
128 is connected to Microsoft box in which ISA is installed and ISA client is running on each machine

all messangers , web cam , voice (Real Time Trafic) is controlled by ISA
and all Port 80 requests (Browsing) is controlled by Squid

so every thingis working well
except MSN
msn is wroking on browser based mean squid based
it is running well
i want to run MSN through ISA not through squid

what i can do to shift msn from squid to ISA