Interrupting startup and creating service??

[blim]

Hi guys been searching around for clear cut answers for these two questions.

1. How do you interrupt the windows startup on XP pro? Ie. Make another program run before explorer and only launch explorer when the program is ended.

2. How to manually create a custom service? ie. Make a program run as a service.

For both of these I'd like to know how to manually do. So NO extra software.
I know there's heaps of software the can make a program run as a service.
There has to be a way via registry, but how?

Thanks