Not getting 1Gbps speed in Office

Ok here is a TestKing question. Juz joking.. haha My office is on 1Gbps connection, using Server 2003 Standard R2 as DC, 1Gbps switch and CAT5 cable. All client using are XP SP2. When i try to copy a 5gb file(single session) over to my side from another client which is only 1 hop away, I'm only using up only 15% of the max speed which work out to be only 150Mbps. Wad could be wrong? HDD not writing fast enough thus bottlenecking?