My memory on XP is now a bit hazy but I think USB plug-and-play support was included from the outset. Certainly with SP1/2, connecting the handset via USB cable should give you notification on the phone of a USB connection, and confirming this should show the storage volumes (internal and external) as two removable drive in Explorer.
Installing Kies also installs the necessary drivers for MTP support, but basic removable storage support is included with the OS.