Hit ctrl-alt-delete at the same time to bring up the task manager.
Click on ther "performance" tab.
If the X2 is being correctly identified, you will see 2 CPU usage graphs. 1 for each core.
Go to your Control Panel, System, Hardware, Device Manager, Processor. Again, if the X2 is being detected correctly, you will have 2 CPU's listed.
If you only have 1 CPU graph in the device manager, or 1 CPU listed in your device manager, your X2 is NOT being detected correctly. It is, in your case, most likely your motherboard not being dual core compatable.
When you installed the new CPU, upon the first boot-up into Windows, Windows should have detected the new CPU, installed it, (it was loading the multi-processor kernel and drivers) and asked you to re-boot to complete the installation. Did that happen?