OS math

I'm new to the forums. I joined because this seemed the best place to seek help for my issue. I have a black Macbook from 2007 with a 160 GB hard drive and 2 GB RAM. I've experienced one hard drive melt down in 2008 and swapped out a broken disc drive.
Right now, my hard drive is full. It's not a big issue. I filled up the external drive I had with time machine back ups, and I wanted to get a new external anyway. The issue is that when I got a message that my start up drive was almost full, and that I was almost out of virtual memory, I went to delete some stuff. I deleted some movies that I hadn't watched in a while, including a blu-ray one that was over a gig in size. Yet, after all of that, the stats for my Macintosh HD read that I only have 157 MB of space left.
I'm not sure how it works to actually delete things because I've put it in the trash and emptied the trash, numerous times. And yet, somehow I don't have any additional space on my hard drive. I'm not great at math, but I can add, and so I guess I just don't get OS math. Any advice would be greatly appreciated.

I'm guessing that your drive is so severely full and short-changing your swap file that, as you are deleting files, the virtual memory is expanding to take up the newly-freed space. One thing you may want to try is flushing your cache files. Onyx can do this.

When was the last time you rebooted your system? A reboot will flush any old unused swap files, that may not have removed.

I rebooted my system and suddenly I had 944 MB, so that definitely helps. Thanks for the tips. I'm going to try the Onyx one now. I also got a new external for school in the fall (LaCie 1TB) so I'll transfer over my hard drive contents and that should free up pretty much all the space.

+1 for OnyX. You should see a big difference because those caches haven't been cleaned in three years