Usually when I talk about system architecture I refer to 32-bit or 64 bit architecture. Broadly I would say it is to do with design and operation of the computer.
I think you are confused with sizes. I would say you have 32-bit or 64-bit architecture size is of no concern.
lol, he's referring to the byte-order his operating system uses when storing data. say a 16 bit integer needs to be stored in memory by a program. It can either be stored in little endian format, or big endian format. Think of "endian" meaning end. so little end or big end. Since a 16 bit integer takes up two bytes in memory--this refers to whether the "big end" (the high order byte) or "little end" (low order byte) comes first in its storage.
Lets store 41654 as an unsigned integer in memory. 41654 is A2B6 in hex, which is two bytes of memory. A2 is the big end, B6 is the little end, so big endian format would store this as A2 B6 and little endian would store it as B6 A2. This is important when reading data which has been sent from another system which may use a different format, or when it's not implicit which format was used to write the data out to memory.
And yes, as far as I know, Windows XP uses the little endian encoding system.