iinstalled 10.6 on top of 10.6.8

[nixphoto]

Bought a used 2009 mac mini loaded with OS x 10.6.8. I put in a new hard drive. I already had DVD with 10.6. I stuck that in. Had planned to follow that up with installation of 10.7. I must have put the adm/password at the wrong place? New hard drive opens to adm/password and for a while the password worked, allowing me to do anything that didn't require adm/password. In finder there was another adm. name. Now it opens to the opening page adm./password page, but no longer accepts password. None of the usual corrections: cmd./s; cmd/r; c ; or others worked. Held the shutdown button down five seconds, unplugged, et al. I need to wipe hard drive and start over. At this point, the computer comes on at the adm/password page, but password doesn't open it anymore. There must be some way to overcome this, i just don't know how. I've looked everywhere. I'm at wits end. Please help.