On the command line a 'shutdown /a' will abort an active shutdown in XP
Appearently the person that did this to you is not the smartest since he did not set the time limit. (30 is default).
To prevent things like this happening take phishphreek80 's advice and install a firewall on your computer.
Also disable unneccesary services. This topic has also been covered to an high extent before so do a search in the 'Microsoft security discussions' forum

Hope that can be of any help
Buenos Dias
noODle