First look at this website so that you may actually get rid of MacKeeper:
Get your install disks out and reload your OS. I would suggest purchasing Snow Leopard from the Apple store and instead. Only $20.
Archive and install from the 10.5 disk, hold c and boot off the disk
BTW you should be on 10.6.8 now minimal, 10.5 is obsolete.
Call Apple for the 10.6.3 white disks.
Backup your data to a storage drive and disconnect, then follow these instructions if the upgrade goes bad.
Although I have not managed (in brief testing) to duplicate this, MacKeeper is known to sometimes too-aggressively "clean" the system, destroying important parts of the system. (In fact, I'd be curious to know exactly what you did so I can see if I can duplicate it in a test system.)
The fortunate part of this is that you can install 10.6 right on top of your existing damaged system and it should (in theory) work just fine. (That's assuming there's no underlying directory damage or something like that.) So, if your computer is capable of running Snow Leopard, get a copy of that:
Once you've got the disc, start up the machine with the disc in the drive and with the C key pressed. Before proceeding with the install, repair the hard drive with Disk Utility. Then proceed with installing, which should leave all your data in place. (Of course, the first thing you should do once you can use your machine again is start making regular backups!)
If your machine cannot run Snow Leopard, call Apple for replacement Leopard discs. They will send them to you for a small shipping & handling fee. However, you cannot install Leopard right on top of the existing system, so you will need to do an "Archive & Install," which will archive your user folder to allow you to get access to your data from the new user you will set up on the new system.