Yes, DC HUB is a securit issue.
I had no end of issues with people intentionally trying to gain access, the number of times I found my system was bombarded and the firewall was recording hits at the HUB port aand also multiple hits were recorded in the router as software firewall ran at the router.
Also mail servers are always being abused, plenty of spammers want to spam mail people from open proxys, always run your smtp & pop server with authentication and preferably through a recognised ISP, I have used Argosoft mail server and its prety good and uncomplicated unlike some I have tried.
You are relying on windows XP firewall, no wonder your being or have been hacked, that wont stop owt for toffee. Kill it off completely and install a 3rd party firewall like zonealarm, which actually represents the least trouble setting up, if you take a little time to poke about ans understand what your doing.
If your router has the machine in the DMZ then take it out of the DMZ and start forwarding ports and generally sitting down and configuring your router, if that has some firewall capabilities then invoke them.
You can do allot security wise and you really should try AVG6 or AVG7 from
www.grisoft.com as it is a very good Anti-virus scanner and removal tool with a good track record. It has saved me a few times from web sites trying to take advantage of exploits in browsers. You could have picked this up by simply browsing...
Run some scanning software like...
Ad-aware from
www.lavasoft.de and grab the Adaware SE
Spywareblaster from
www.javacoolsoftware.com
Spybot Search & Destroy from
www.safer-networking.org
and they will clear your system, thay all have proven track records and do not put any spyware on your system as some spyware removal tool competitors claim, they do however clearout allot of nasty stuff, especially after being run for the first time, be in for a shocker. my first time cleared out opened my eyes.