All of a sudden, entire ranges of performance counters vanished from my system. I first noticed when my taskbar traffic monitor stopped working today, and looking closer, I found that stuff like \\MACHINE\Network Interface(*)\Bytes Sent/sec simply disappeared from perfmon.
I eventually figured out that a brand new DWORD value called Disable Performance Counters, set to 1 had magically appeared overnight inside HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Performance. Removing that (and, of course, rebooting) made the networking counters reappear, but I'm still trying to figure out what happened.
I'm betting my money on the new .NET CLR update, which created a whole new set of counters, but I still suspect SP2 - it just might have something to do with this. I wonder, however, how many people will notice this sort of thing and realize exactly how infernally buggy the system must be for it to happen.