The Windows XP operating system was originally designed when only 32-bit computer processors were available. The advent of the 64-bit multi-core processors saw the release of Windows XP for those CPUs. Windows XP Pro for 32-bit systems is designated as x86, and Windows XP Pro for 64-bit system is designated as x64.

History

Windows XP was released in 2001 before the advent of 64-bit computing. As such, the architecture of Windows XP Pro is based on 32-bit processors. The rising popularity of dual-core processors saw the release of Windows XP Pro x64 and the designation of the original Windows XP Pro as x86.

x86 versus x64

The obvious difference between x86 and x64 is the architecture of the operating system. An x86 operating system is only able to utilize a 32-bit CPU, whereas a x64 operating system can utilize a 64-bit CPU. Another difference is support levels of system RAM. Windows XP Pro x86 can only support 4GB of memory whereas x64 can support 8GB of memory.

Compatibility

Windows XP Pro x86 is compatible with most modern processors, including 64-bit CPUs. Installing Windows XP Pro x86 on a 64-bit system is no problem, and the operating system will simply make use of a single processor in the case of a multi-core processor.

Performance

Windows XP Pro x86 obviously is not able to take advantage of the performance boosts that a 64-bit multi-core processor provides to computer systems. Since the architecture does not support such processors, performance compared to x64 operating systems utilizing these CPUs will be slower especially for multi-tasking and graphic intensive tasks.

Misconceptions

Windows XP Pro was originally designed as an x86 system and as such does not identify x86 in the product title, as it does when in the product title of Windows XP Pro x64. That the architecture is not specified does not indicate that the system is not an x86 system. Rather, any Windows XP Pro title that does not specify the CPU architecture can be assumed to be an x86 system.

Source:

Microsoft.com: Frequently Asked Questions About Windows XP Professional x64 Edition

x86 CPU Guide