As Jacorre says, more RAM would help, although if you can't afford another hard disk I'm guessing you probably won't be able to afford more RAM at the moment either. The way Photoshop works is that it uses whatever RAM is available and then, once that's full, it starts using the empty space on your hard disk as temporary storage (the so-called "scratch disk"). The area is freed when you quit Photoshop, but while you're working you need a reasonable amount of free disk space.
The only other way is to free up some space on your hard disk. Sometimes it's not immediately apparent which files and folders are hogging the most space, but there's a utility called
TreeSize that you can use to give you a good picture of how your hard disk is divided up. You can then choose what to remove to free up space. For the most part, brushes and custom styles don't take up that much space - you might find that you've got some huge temporary files on your disk somewhere, or there may be some space-hogging, seldom-used programs that you could uninstall. You can download TreeSize from here:
JAM Software - TreeSize Personal - Hard Disk Space Manager that shows folder size and many more statistics