About Trinoo_Master on 27665 tcp

On my Cisco Router, I do a nmap from outside on the Internet. The result 
is:

" Interesting ports on *.*.50.1:
Not shown: 1676 closed ports
PORT STATE SERVICE
23/tcp filtered telnet
135/tcp filtered msrpc
1524/tcp filtered ingreslock
27665/tcp filtered Trinoo_Master

I am worried about the last two entries. The last nmap was done in Feb 
this year and I have confirmed that the two ports did not exist.
Though the state "filtered" is a solace but I am still concerned. How 
can O be sure that the system has not been compromised?

Also the current IOS Version on my Router is 12.4. It was the same case 
when I was using older v 12.2 on another router, so I thought maybe, 
it's an IOS issue and I upgraded my Router to 2811 with IOS v 12.4.

But as soon as I plugged it into the circuit, I realised the nmap again 
gives the Trinoo_Master entry with state as filtered.

Where could lie the problem. Is it with my firewall configuration behind 
the router?
Or is it my ISP trying to block access to these ports through my home 
connection?

------------------------------------------------------------------------
This List Sponsored by: Cenzic

Need to secure your web apps?
Cenzic Hailstorm finds vulnerabilities fast.
Click the link to buy it, try it or download Hailstorm for FREE.
http://www.cenzic.com/products_services/download_hailstorm.php?camp=701600000008bOW
------------------------------------------------------------------------