How do I know which Windows bit count (32 or 64) I have?

XP

Click Start, and then click Control Panel.

Then open System.

Then the System Properties opens.

For a 64-bit version operating system: Windows XP Professional x64 Edition Version < Year> appears under System.
For a 32-bit version operating system: Windows XP Professional Version <Year> appears under System.
Note <Year> is a placeholder for a year.

Vista

Click Start, and then click Control Panel.

Then open System.

The operating system information is displayed as follows:

The bit version is listed on the right of System type: