Its two greek letters together, Chi and Rho (X and R) that to this day translate into Christ in Latin and Greek rite Catholic churches. Why the X (Chi) was ever substituted alone, to represent the entire name of Christ by itself, I don't know.
To us its Christ and Christmas.
Hope everyone is having a blessed Advent season.
Actually, the Greek letter Rho looks like the English letter P, not R.
I run Windows XP (Christ Professional)
Microsoft products are given codenames during development. The current trend is to name them after mountains (the next version of Windows is Longhorn, while the codename for Windows XP was Whistler) but once upon a time they were codenamed after cities, so Windows 95 was Chicago. The Windows NT codebase, which was integrated into Windows XP, was codenamed Cairo... which is where the name Windows XP comes from. XP, Chi-Rho, Cairo. Get it?