Why is my laptops memory so full?

I bought an Acer laptop with windows vista home premium, 120gb HDD, 1 GB DDR2, 1.73 GHz, 533 MHz FSB, 1 MB L2 Cache, about more than a year ago. Right now when I use the laptop, this small window pops out on the botom right corner of the screen and it tells me something about acer is almost out of space. It wants me to try and delete old or unnecesary files. well the point is that I want to kno if anyone can tell me why its almost out of space because i Know that i dont have more than 4 gb of music, and i dont have more than 1 gb of pictures and i dont have any videos at all. What might be taking up all that space?? What can i do to solve that problem?