Yes. 120 Gb would be the size of the hard drive, where your documents & programs are stored. You will also see a figure for RAM which is another type of memory; it's what the computer uses to actually run the programs. This could be from about 512 Mb (half a GB) up to 2 or 3 Gb or even more. I doubt you'd find a new laptop on sale with 120 Mb of RAM.
If you're going to be running Vista you need atleast 1Gb of RAM, and preferably 2GB