irule to distribute based on user agent

Try:

when CLIENT_ACCEPTED {
    set default_pool [LB::server pool]
}
when HTTP_REQUEST {
    if { [string tolower [HTTP::header User-Agent]] contains "blah" } {
        pool ms_pool
    } else {
        pool $default_pool
    }
}

Change "blah" to a unique value in the IE User-Agent header, and change ms_pool to the name if your MS pool.