Service Level Agreements

I’m working for a relatively new company.
We install, fix an support all kinds of IT related product.
It varies from hubs, switches and routers to software related products from Microsoft and Novell.

Right now I am working on a SLA for supporting these products.
I find it quite difficult on where to start and what the SLA should contain.

Can anyone help me by sending me an example or template SLA? Or perhaps a link on the internet containing information on how to setup a SLA.