Apparently, there is a fairly simple test to see if Windows is running on two processors or with hyper-threading: Bring up the Windows Task Manager (Shift + Ctrl + ESC) and go into the 'Performance' tab.
Just to cover various single / dual CPU setups with/without hyper-threading, I'll compare various setups:
If you have one CPU without hyperthreading such as a Celeron, there will be one CPU usage history graph. You will also get only one graph if you have a hyperthreading capable processor with hyperthreading disabled. Apparently, I have seen Dell PCs shipped with hyperthreading disabled by default, which must be enabled in the BIOS setup.
For a PC with a hyperthreading / dual-core CPU (such as a Pentium 4 HT or AMD X2), there will be two CPU usage history graphs. If you have a look in the 'Processors' section in the device manager, it will also appear like you have two CPUs installed (same CPU listed twice). When a single-threaded application is running at full processing power, one of the CPU usage graphs will be maxed with the other showing idle usage. However, if you have a multi-threaded application maxed out (or two single-threaded applications using max processing power), both graphs should max out.
If your PC has two XEON (or other) CPUs that are not hyperthreading capable (or with hyperthreading disabled), there will be two CPU usage history graphs in the task manager also; each showing the CPU usage for the given processor.
Finally, if you have two XEON (or other) CPUs that with hyperthreading enabled (or are dual-core), there will be four CPU usage history graphs in the task manager; two graphs per processor.
A simple way to test if a CPU intensive application is single or multi-threaded is to get it to run at full processing power. For example, if this is an MP3 encoder, give it a couple of songs to encode. If only one CPU usage history graph is maxed out in the Task Manager, then the application is either single-threaded or set to run on a single thread. However, if two or more CPU usage graphs are maxed out, then it is multi-threaded.
Going by your PC set up, you should have 4 CPU usage history graphs in the Task Manager; asssuming the CPUs have hyper-threading capability.