The importance of knowing whether or not you are running a 32-bit or 64-bit version of Windows, makes it easier for you determine what hardware/software is compatible with your computer operating system. Follow the instructions below to see which version you are running.....

Windows XP or Windows Server 2003

Click Start>Run

Type sysdm.cpl then Click OK

Make sure you are on the General tab

Look under Operating System

If the operating system has an x64 in it, then you are running a 64-bit version. If you do NOT see this, you are running the 32-bit version.

Windows Vista

Click Start.

Type system in the Start Search box.

Click system in the Programs list.

Look at the operating system.

The System type under System value will show either 64-bit or 32-bit.

Alternatively, clikc this link for full instructions from Microsoft