Setup a PC to watch (sniff) the traffic coming into your Xbox, if you see an increase of traffic from one host in regards to that, simply block their source IP on your Firewall. If you’re really adventurous, find out what ISP owns that IP address and contact them. Most ISP’s acceptable usage guides, forbid interfering with other internet users via DoS attacks. Contact them, but you’ll need evidence, easy enough..

Now the the bugger would be if multiple PCs are sending this traffic to your Xbox, it might take you a while to block them all.

Now what can Microsoft do? Well if it’s a PC sending you the traffic, not much, unless you can somehow determine the IP of the Xbox (and maybe some other data) from behind the same network that PC is attacking you from (see my ISP tip above).

Sniffer Traces (search for a program called WireShark) are easy enough to setup, and you can save the conversation easily (you might need a hub to do this), you save the conversation to a file.
Microsoft might allow you to send that trace to them for analysis.