My computer at work has an 8.5Gb hard drive. It suddenly flashed up a "low memory on C Drive" warning. Lucky, really, as the IT engineer happened to be on site.
Apparently being down to only 23Mb of available memory from 8.5Gb is NOT a good thing.
He cleaned 1.5Gg of crap off it (mainly Windows error reports and bits of random Microsoft bloatware, I think) but we are looking at a second drive, internal or external for storing images, etc.