Ok, now at the top of the window is a menu bar.
In that will be Utilities. From that drop down menu select Disk utility.
There will be a list of hard drives and optical drives on the left of the window, if your drive is in that list then click on it once to highlight.
Now click on the First Aid tab, at the bottom of the Disk Utility window you will see a button called Repair Disk.
Select that by clicking on it. It will either return that everything is okay or that some things were wrong and they were repaired or it found something wrong and it can do nothing about it.
If everything checks out okay or if it does the repairs you then want to repair the permissions.
There will be a button close to the list of drives that says Repair Permissions. Click that to selected and give it time.
There may be a long list of permissions that repairs or there may be only a few.
There will also be some that say SUID that will not be repaired if you have 10.5 or above.
Once you have completed this then you can quit Disk Utility and the installer.
Do Not Install Anything.
Simply restart your machine by selecting it from the menu bar.
Try that and see if it will now boot normally.