administrator name and password

I can't download any new software or make any changes on my computer because that requires an administrator name and password. Somehow, I am only a standard user on my computer and I am the only account on the computer (besides system administrator (root). I've used my install disc multiple times to change my password, but that doesn't work when I enter my user name and the new password when the computer asks for an administrator name and password. It just says password is invalid and administrator name and password is required. any warranty and technical support has expired and I'm at a loss at what to do. I really need to download new software and need to somehow recover the original administrator name and password and change my standard user account to an administrator account. I'd appreciate any help or advice!