Zarya (the first module, launched in 1998) was pressurized and could support a crew short-term-- the crew of STS-88 (which launched Unity, the second module added to the ISS) entered the station and began unstowing gear and stuff. The ISS couldn't support its permanent crew until Zvezda (the station's service module) launched in 2000, though-- Zvezda contains the station's primary life support equipment and contained crew facilities like toilets and bunks.
