Why does my ACER PC computer have two drives?

I've had this computer for a little more than 3 years. It's Acer, and it runs on Windows Vista. It says it has 160 GB (Which sucks) of SATA hard disk. I have no idea, what that means, but I'm guessing it's memory. It's currently running low on Memory. However, there are TWO drives on my computer. One is called ACER(C:) which has ALL of my files, music, pictures, etc. and it has 1.49 GB free out of 69. 7 GB. The other is called DATA(D:) and it has absolutely nothing on it, and also has 69.5 GB. Why? and If there are 160 GB of memory in this computer , WHERE are they? Is there a way of merging these two? Please help.