After quite a long blog on managed semaphores this would be quite a refresher. :)
When trying to connect your XBOX 360 to your vista with Media center things can get pretty interesting. There are quite a few posts that would tell you trouble shooting issues. This is pertaining to IPSEC and a Vista machine that belongs to a domain with pretty stringent IPSEC group policy and you bring your laptop home but your xbox just cant connect to the vista machine. For direct connectivity refer http://support.microsoft.com/kb/932161 and other posts on support.microsoft.com.
If you get this message in your Security Audit log like "dropped packet in clear text" where the IP is that of your xbox then you could check these steps:
1. Check if your Xbox connects to Xbox live without any issues.
2. Check if your machine also connects to the internet without any problems over the same .
3: Open advanced firewall settings and navigate to Monitoring->Connection security rules and check if your home lan net is exempted from the IPSec rule policy. usually the range is Endpoint 2 IP address:192.168.0.0-192.168.1.255.
4. Finally check if your xbox and your system lies in these ranges for the policy to permit.In my case i had to change my router address to these ranges..
There were many posts the suggested turning of IPSEC but this seems to be a better way IMHO :)