As I stated, the info reflected under System...refers to the O/S.
The info under computer refers to your CPU.
CPUs have all been 64-bit for much longer than you think, but it's the operating system that determines whether a system is running at 32-bit or 64-bit.
There is a 64-bit version of XP, but it is used primarily by enthusiasts and others who wanted to employ more RAM
than is supported by 32-bit Windows XP.http://www.microsoft.com/windowsxp/64bit/default.mspx
Both Vista and Windows 7 have 32-bit and 64-bit flavors, with the intent of providing users more choices and not necessarily creating unnecessary/undesireable hardware upgrades to use either Vista or Windows 7.