Bueno, acaban de solucionarme el problema. Aquí les dejo esto por si les sirviera en un futuro: ¡La culpa la tenía el cortafuegos! ¡Resulta que el debug usa tcp/ip!
Cita: Java debugging uses TCP/IP sockets by default (even when debugging on the same computer). Software firewalls can block such TCP/IP activity, thus making debugging fail.
I can't say for sure if that is your problem, but you should check if you have a firewall running (don't forget that windoze xp has a built-in firewall that may or may not be enabled) and try shutting it down to see if that resolves the problem.
I seem to remember that SP2 of windoze xp enabled some more strict policy in the firewall that caused some people problems. So if you recently upgraded your windoze it might have changed your firewall settings.