Running as administrator

I am writing a utility to secure files and folders. I want this to be compatible with both XP and Vista. Unfortunately, in order to access certain folders in vista I would need administrator privileges. Microsoft recommends having a service in the background with administrator privileges, then simply calling into that service from the application when necessary, rather than elevating the application to administrator. This also prevents the annoying UAC prompt.

My question is, how can I write the program in such a way that it will work in both XP and Vista? The service in the background isn't really necessary in XP, i'm thinking. Also, does anyone have any ideas on how to implement this service/application writing? I don't have much experience with services.