1. Open Performance Infomation and Tools (Right click my computer and select "properties" Click on Windows Experience Index)
2. Click "View and Print detailed performance and system Information".
3. In the System section, you can see what type of operating system you're currently running under System type, and whether or not you can run a 64-bit version of Windows under 64-bit capable. (If your computer is already running a 64-bit version of Windows, you won't see the 64-bit capable listing.)