The Compaq EVO W6000 is capable of 2 processors, doesn't mean it actually has 2 processors. Also, it supports Hyper-Threading, not Dual-Core. That basically means that the processor can process 2 threads at once, but isn't actually two cores.
Since you are seeing two CPUs in the device manager, it means one of two things: 1) you have 1 processor with hyper-threading enabled or 2) you have 2 processors with hyper-threading disabled.
You can check your BIOS to see what processors are installed.