There are different ways to create a Service Monitor in SCOM 2007 R2, I find the method outlined in the pdf attached to this post the best way to do this (using the Authoring template). Once you have created the Monitor you can change the priority/severity by looking at the monitor in the Authoring Console via Availability, Basic Service Monitor and changing it here using an override.
Other ways to create this basic service monitor would be as follows:-
In SCOM Management Console, Authoring tab, Management Pack Objects, Highlight Monitors and Scope for Windows Computers,Expand Entity Health under target windows computer, Right click on Availability and choose create Unit Monitor, run through the wizard.
Or via the attached doc (similar to above but not using target first)