You can get to a command prompt from the installation DVD. There are 2 ways.
1. Boot from the DVD, on the first window, do not select your language, hold down shift and press F10, it will bring up a command prompt window.
2. On either the 2nd or third screen when booting from the DVD, in the bottom left it says repair my computer. you can click there and it will do startup repair, which I would try. or click through to other options and one will be a command prompt.
From what little I know about the registry, importing a clean registry will not fix your problem. You have software installed which write registry keys, and they will not be changed. Merging a new registry will only change the keys included on the DVD and not all of the ones you have changed. You would have needed to do a registry backup before you took ownership.
Your best bet would be to do a system restore from the DVD or try the startup repair from the DVD.
Personally, if it were me, I would do the startup repair and system restore from the dvd. If those did not work I would try a repair install. If that didn't work, do a clean install. The time you have already spent, you could have done the clean install and have a working computer. Use the Hirens to get your personal files out first and try the repair from the installation DVD. If you do the registry change, you will only change the keys that are on the DVD. Any keys or values that were changed will not be affected. What I honestly believe is that if you manage to fix it to where you can boot, you will still have a problem install. You can spend more time trying to fix something than it would take to do a clean install and be done with it and have a perfect install. That's my personal opinion based on numerous hours trying to fix something I have messed up. Don't feel bad, most of us have done the same type stuff and worse. But, we learned a lot about things from the experience. If the steps Greg linked to on troubleshooting Windows 7 don't work, a clean install is your only option, I believe.
EDIT: I am far from an expert on Windows registry. I am only telling you my opinions.