Difference between 32 bit and 64bit?

This applies to Windows XP and all 32bit or 64bit Windows Operating systems:

32bit Windows Operating systems are made to be used with 32bit motherboard/processors.

All 64bit Windows Operating systems can only be used on a 64bit motherboard/processor and will not work with 32bit motherboard/processor architecture. 64bit Windows Operating Systems is also the only one made to utilize all of the processors on multi core processor (dual core, quad core, etc) systems.

All Windows Operating systems are designed to function with a specific hardware architecture design.

*some 64bit processor/motherboard combinations have a backward compatibility that allow you to use a 32bit operating system on a 64bit system by disabling 32bit. When you operate a 64bit processor in 32 bit mode it turns off some of the processors. This means a dual core will work as a single core, a quad core will work as a dual core (you can never get more the 2 processors to function in 32bit mode, even if there are more then 4 processors on the same processor die.)