Replying to FUCK THIS, HELP ME

So, something went wrong with my MacBook a few weeks ago and it would not start up. I brought it to the campus computer help center, and after hours of searching, they figured that the login window was crashing. Their solution: reinstall leopard. Before this incident, I had 25GB of space on my hard drive, and life was good. After reinstalling leopard, making sure it turns on now, and taking it home, I realized that I had lost OVER 20GB OF SPACE. Where the fuck did it go?? How can I get it back?? Now, I have 1GB left, and I need help. Are there some kind of archiving files from the reinstallation that I can delete now? Help me damnit!