Losing memory on hard drive, any ideas?

Generally speaking, this is a normal attribute of running your machine. The operating system stores information on your hard drive as you're browsing the internet, working in Photoshop, etc. Even more hard drive memory might be used if you don't have enough RAM, since the operating system then needs to use the hard drive for temporary storage. And if that happens, your machine will be running as slow as molasses (hard drives are much slower than RAM), which is why it's important to have an adequate amount of RAM.

Rebooting the machine will restore a good chunk of the "lost" memory, but never all of it (even if you completely clean out all of the folders where cache files are stored).

Given how inexpensive storage has become, you may be able to find a friend who upgraded their hard drive and doesn't need the old one. Even a five year old drive could easily have 120 GB or more on it, which would almost triple your storage capacity. I'd suggest finding that sort of inexpensive (or free!) solution if your drive's space is approaching zero.