You always want to use 64-bit Opus on a 64-bit OS.
The x64 version of Windows XP is for x64 processors, the same as the x64 versions of Vista, Win7 and Win8.
(The Windows XP Itanium version is something different. Virtually nothing runs on Itanium, which doesn't matter as Itanium ended up a dead-end and stopped being used years ago. x64 is the 64-bit architecture which we all use; Itanium can be forgotten.)
As long as you have XP x64 with SP2 installed, or XP x86 with SP3 installed, the appropriate 32-bit or 64-bit version of Opus should install on it, as per the Minimum Supported versions of Windows section at the top of the FAQ which the dialog in the root post points to.