Ok, thank you for the additional information!
I'm guessing that your brother found a folder on your hard drive called "Previous Systems". It should be at the root level of your Macintosh HD. If it is, look inside it. I'm assuming your system is running the way you want it now and you have all of your applications and files. If not, you might check in the things you're deleting to make sure you still have them.
The Previous Systems folder contains a lot of files. A list of them is here, under "Files and folders that are moved into Previous Systems":
The one that you need to be most careful of is the Users folder, which contains any backed up user data (e.g., Word files) that wasn't transferred over during your operating system install. This folder may not exist, in which case you should be fine. Check and see if there are any files in the Previous Systems folder that you recognize that you don't have already, but if you don't see any then it's okay to delete the rest of them. Make sure to empty the trash when you're done, and that should help a lot.
Let me know how that works for you. It's good to have at *least* 1 GB of free hard drive space, so if deleting the previous systems doesn't give you that kind of space then I can lead you through reducing the size of your iPhoto library.