UPDATE:

This has been fixed in a recent hotfix (951979).  This also fixes a number of other issues.

http://support.microsoft.com/kb/951979

If you cannot apply this hotfix now for whatever reason...read on...

You have probably seen this alert in your Operations Manager console, after upgrading to SP1.  This monitor has been identified as a bug, and the workaround is posted all over the blogs.  If you're like me, I generally take the advice of anything posted on Operations Manager Product Team Blog.

Here is the product team's posting for this workaround.

http://blogs.technet.com/momteam/archive/2008/03/01/performance-module-could-not-find-a-performance-counter.aspx

There is only one thing I would add to this.  It's a good practice to save overrides to your own management packs.  As you probably know, if you override a rule/monitor by right-clicking and traversing > Overrides > Disable the Rule, this action will always save the override in the Default Management Pack.  This is why I advise customers to disable a rule by traversing > Overrides > Override the Rule, set the "enabled" parameter to false, and place the override into a MP other than the Default MP.   I always advise a customer to create a MP, and never use the Default Management Pack.  And this case is no exception.

In fact, in my environment I have a MP called "Override - Product workaround and bug".  As you might suspect, I use this MP specifically for overrides pertaining to workarounds or bugs in the Operations Manager product or Management Packs themselves.  This helps to keep these types of overrides organized in one container, so as not to mix them up with overrides that were decided upon by [your] organization.