This is a bug in SP1. After installing SP1, users will see this error flooding the Alerts screen (and email if they have a subscription enabled for it)

There is currently no hotfix or patches for it yet. There is a work around, however, please follow the following steps to overide this error until Microsoft release a fix for it.

In the SCOM 2007 Console click on Authoring
Expand Authoring - Expand Management Packs Objects
Select Rules
Enter "Performance Module could not find a performance counter" in the Look For: search box
Click Find Now.
Scroll down the results and look for "Type: Health Service (2)"
Select "Performance Module could not find a performance counter"
Right click, choose "Overrides", "Disable the Rules", "For all objects of type: Health Service".
Click Yes

Go back to the Monitoring tab.
Go to Active Alerts - Highlight all "Performance Module could not find a performance counter" and close them

That should be it, until Microsoft release a patch for this.