Good info on perfmon counters from ... http://www.itexpertmag.com/server/set-up-event-subscriptions-in-windows-2008
Processor \ % Processor Time.
This counter is the overall measurement of when a server
is actively processing useful work. It measures when the processor is doing something other than the System Idle Process. When this counter is high, this means that the processor is actively processing useful work and you have efficient utilisation; it’s only a concern if users complain of poor server performance that corresponds with periods of high processor use. However, when this counter spikes to 100% – or 50% in a dual-processor system –
it can mean that one process is consuming an entire processor’s resources and needs attention.
System \ Processor Queue Length.
This counter shows how many instructions are currently “in line” for attention by the processor. When this counter goes much above zero, it is often an indication that the processor cannot keep up with the workload you are asking it to perform. A high count here can indicate that you need either a faster processor or fewer services running on the server.
System \ Context Switches / Sec.
A context switch occurs when a processor “switches” between which waiting instructions it is processing. As processors are only able to process a single instruction at a time, context switches give the illusion of multitasking. High levels of context switches are problematic, because of the resource overhead involved with swapping out what the processor is working on. In situations where too many actions are being required out of the processor at the same time, this can be a very high number. Typically, you’ll see very high figures with very old applications or on Terminal Servers where many users are running many processes at once.
Memory \ Available MBytes and Memory \ Pages / Sec.
These counters are useful for determining memory use on the server.
When a server processes its workload, it loads elements into memory for processing. When that memory begins to fill up, pages are swapped out of RAM to the disk. Since the disk subsystem is significantly slower than solid-state RAM, swapping usually involves a reduction in overall performance. Thus, the count of Available MBytes should be a number greater than zero, while the count for Pages / Sec should be a relatively low number.