What is this?

I was watching TCPDUMP display traffic on my LAN at home last night and
saw this scroll by.

0.210654 ip68-2-176-35.ph.ph.cox.net > MICROSOFT-DS.MCAST.NET: igmp v2
report MICROSOFT-DS.MCAST.NET [ttl 1]
23:39:00.221615 ip68-2-177-28.ph.ph.cox.net > 239.255.255.250: igmp v1
report 239.255.255.250 [ttl 1]

Being that is clearly said microsoft I questioned why when my laptop was
running Linux. My question is what is that mcast.net and why is it
showing up on my "LAN's Freeway"? My roommate runs XP and I think the
computer in the living room is W2K but if they are invoking it, why?

Just curious...