HELP! Can't log into my Windows Xp Pro SP2

Ok I know I'm a total idiot but here I go so I got my computer from a friend apparently out use to belong to a company. I logged on no problem and I started to clean it out I did not want any of there files and I have no disk do I did out manually. Anyways I deleted users it didn't say out was the administrator renamed the computer and rebooted it. Now when I try to log in the old user Id is on there which I deleted I don't have a password for that user and when I type administrator and push enter out doesn't work I tried password the password nothing I tried safe mode command prompt f8 f5 f everything and nothing the only option I have on the login is shut down our restart out has insert I'd and password what do I do pleaser somebody help me???