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I used to use ICS with no problems on Win98SE, I assume you've used the information from http://www.xbox.com/en-GB/live/start/connect/sharedpc/ ?
From http://www.xbox.com/en-gb/live/start/connect/sharedpc /ics.htm select your OS and double check everything.
If all that is set right then the best people to contact is Xbox Live support - most ISP's don't seem to know anything much about Live - but don't email them, ring. Email responses in my experience take about a week.