I think that Simons possibly pointed you in the right direction, but as he often does at work hes taken a wrong turn

You have SP2 on dont you Hookie ?
Maybe if you disable that and see if it allows it to synch and if it does then its just a case of configuring XPs built in firewall.
The firewall can block LANs via ethernet and USB so presumably it can do that on the COM ports too
