Error receiving multicast packets on PC

[cs02lk1]

I have a really strange problem with my PC and I don't even know how to explain it.
I run an instance of VLC which broadcasts the data of a video file on a multicast address and then I run a different instance on the same PC for receiving and displaying the video.
This worked fine and without any problems when I was working on Windows XP Home Edition SP2. Last week I have bought and installed Windows XP Professional Edition SP2 OEM and when I tried to do the same the receiver did not accept anything.
After numerous of tries and experiments I accidentally found out that when I use a Packet Sniffer Application the time I transmit the video everything works fine again. but only when the Packet sniffer (Ethereal) is active and sniffing packets.
This happens with everything I try --> VLC, JMF, two manually created java classes (sending and receiving)

Does anyone has a hint about what is going wrong?

My PC :
HP nx9420
Windows XP Professional SP2
Kaspersky Internet Security 7
Windows Defender

Anything else you need please let me know

Thanks

[Teta]

It could be that your client under the "new" OS doesn't respond to IGMP queries like it should, that's what I have with my new Dell Latitude D-420 laptops. The D-820's with comparable OS configuration work fine.
As I didn't found anything on the Dell website about it, and I read more posts with people having this kind of trouble, I guess going through the Microsoft knowledge base might be a good idea.

Teta

[cs02lk1]

Yes but how do you explain when I sniff the packets with Ethereal Packet Sniffer everything works just fine? The sniffer just collects information about every packet sent from my interface card.