Macbook hard drive space goes down automatically?

So recently my macbook has been really buggy. I'm running really low on space on my macbook, about 600MB. But my hard drive space will slowly go down automatically to zero kb, in a span of an 30 minutes. And when I delete things, it still stays at zero kb. I restart, and it does the same thing..
And when it goes down to zero kb, a window pops up every 30 seconds, pausing one of the applications I have open and forces me to force quit something..
How do I fix this?

It sounds to me like you are loosing space because of the virtual memory files created by OS X. As programs are being used OS X writes some data back to the drive for later access if it is needed. This article might explain things better Mac OS X: Reading system memory usage in Activity Monitor. Pay particular attention to the segment on VM Size.

Here's what I suggest. Restart your Mac and note how much free space is on your hard drive. Then launch Activity Monitor. Select the System Memory tab and notice the number for VM size. That number will likely grow as you go about your activities.

When you experience the problems you describe it is likely that this file will have used almost all of the available space. If that's the case you essentially have two choices: Delete some stuff from the drive to make more space available or install a larger capacity drive. 600 MB is not really a lot of free space. After a days use my MB Pro with 4 GB memory has a vm file that is just over 1.5 GB.