How to combine two hard drives? (with software)?

ive opened up my computer, its a sony vaio 530g running windows xp home and there is ONE hard drive inside. the hard drive has 160 gb, but the computer sees two hard drives: 15 gb and 128 gb. since most programs automatically install on c: (15 gb) without prompt, along with windows update, i cant install anymore programs with 431 mb left on c, but 113 gb left on d!! how do i let the computer know its only one? or at least trick it?