using system user to execute a file

[samibami]

hi there,

i'm not sure if this is the right forum for this question.
i need to execute a file on win xp's system user.
i noticed that it can be done by adding a schedule task to the xp's task scheduler.but how can i do it without adding
a schedule task ?

if i'm not clear enough here's another example of what i need:
i'm using a dynamic dns utility (

http://www.no-ip.com)

that has an option that ables it to run as a system service.
i realy want to know how to do that!

10x again