Service Level Agreement

I manage a Desktop Services Help-Desk and need a simple but effective SLA. I support PC hardware and software including ID creation, but NOT education (‘How do I’ type questions).
I am finding it hard to devise one simple document. All the internaldepartments have different needs which would ultimately require a very large un-manageable document. Has anyone got or seen an example of a good working simple SLA ?
Many thanks,
Kevin Gough.