Normally what I do is create a model in ArcMap/Catalog then export it out to python code, then setup a .bat file that references the python code to run at a particular time. This seems to work best for me, I did try setting up a batch file that references a model directly, but it wouldn't quite work correctly and I wanted to add some logging stuff too.
So anyway once you have exported your script out, you need to make sure that the paths to the data are all correct and the python script runs from ArcMap/Catalog correctly. After this script is setup you need to create a .bat file and then put a line of code like this:
c:\python26\arcgis10.0\python �??c:\script\SimpleModel.py�?�
This will open up python and then refers to the script to run.
After this is setup, all you need to do is setup a basic task in Windows scheduler to run this bat file at a specific time.
What part of the process is failing for you, you may want to put some logging in your script to see where it is failing.