reinstalled OS X discs and cannot access some user's files.

I am the type of user who makes decisions after understanding about 5% of what I am doing. I had previously accidentally erased applications, then reinstalled only the first of two OS X discs and upgraded OS through free Apple upgrades. It seemed easy. I realized I did not have iphoto, so I installed both discs today. This time it was like my computer did not know me anymore....had to set up a new password, etc. Now I do not understand how to get my (and other users') files out of "previous system". Also, do I need to download OS upgrades again...aren't they still in there somewhere? I also fear that I am clogging up my disc. I really appreciate your help!