If you have another mac at home, I recommend using the OS X Recovery Disk Assistant to clone the recovery partition. Then just insert the USB into the non-functional Mac and boot from the USB, as this will allow reinstallation of the OS.
Tip: only use these steps if the built-in recovery partition is not accessible. After taking these steps I would strongly recommend that you upgrade your OS (preferably to Mavericks) if you don't want this to happen again.
There is one troubleshooting step that OS X shares with the classic Mac OS, removing preference files. But on OS X there are TWO folders:
/Library/Preferences and ~/Library/Preferences
--The latter being in your user/home folder--
Since the issue doesn't exist under another logon it is specific to that user folder. SO there is something specific to that user that is causing the problem. This takes the /Library/Preferences folder out of consideration for troubleshooting preferences. The first thing I would do is use a utility like Onyx to verify and clean the system. This includes verifying preferences files. After that rename the ~/Library/Preferences folder to something like
~/Library/Preferences-old
You'll probably have to do that in Terminal and use sudo as those files are owned by another user on the system. then create a new ~/Library/Preferences folder and reboot.
If you can now log into that account it is a simple (HAH!) matter of putting preferences files in the new Preferences folder and rebooting till you find which one it is.
No guarantee, though, that it is a preference file, it could be a startup item. So do the same thing with the ~/Library/LaunchAgents folder.
The other thing to consider is that it might be easier to just create a new user folder. Copy the things you need out of the old user folder and delete the old user. That could save you a lot of troubleshooting time. It would be my preferred way of doing it (after running Onyx) as I care less about what happened than just finding a fix that gets me up and running more quickly.
Best Answer
Sadly, this is a very broad issue, and unfortunately it's difficult to solve over the internet with so little information. I'll give as much help as I can, but you're probably best following online guides until you can come back with more information. MacWorld has a good guide, as does HowToGeek and a host of others. Given what you've done so far, I'll try to help, updating this post as you complete steps. Comment and let me know what the results are.