is there a router connected to the modem ? as in cablemodem>router>computer?
if not , you really SHOULD, it will protect it from a lot of crap that could attack the computer directly
In theory this is what I would do
a) call cabletica and confirm that the service is enabled, tell them you paid RACSA etc blabla
b) once they confirm just plug the computer in, BUT make sure that the windows firewall is ON (this is an absolute minimum)
to do that follow this instructions
http://www.microsoft.com/windowsxp/using/networking/security/winfirewall.mspx
c) that should do it
if it doesnt work (very unlikely but then again , I have never used cabletica) it is possible that they allow service based on the MAC address of the computer (think about that as the serial number of the network card....cabletica would have your account and the 'authorized' mac address ....since each computer has a different MAC address....then you would have to call and tell them 'my mac address is now blablabla')
but I wont get into those details unless you say that they require that