No login window, just a blue screen!

Okay, I love customizing my Mac and tinker with its preferences so I guess I had this one coming, although it's my first major mistake.

I'm running Mac OS 10.5 and I downloaded a loginwindow.bundle from deviantart to customize my login window but I realized too late that it was for Snow Leopard. What I ended up doing was:
- I replaced the original loginwindow.bundle with the one I downloaded; and
- I replaced the original com.apple.loginwindow.plist with the one included above.
- Thankfully, I made a backup of the original files which I kept in my Documents folder.

I then logged out and ... the login window never appeared, just a black blue screen and that round loading thingy at the bottom. I'm sure that's the only cause of my problem. (If it helps, I don't have autologin enabled, so I have two accounts to choose from before I can login.)

I've tried booting in single user mode and in safe mode but neither works.

I've narrowed down my options, and if anybody can help me, I'd REALLY appreciate it:
1. Use some commands in single-user mode to replace the new loginwindow.bundle and loginwindow.plist with the old ones, which are in my Documents folder (like in Terminal). Thing is, I don't know how to do this, if ever it's possible; OR

2. Use my Bootcamp partition (thank God for Bootcamp!) to manually do the abovementioned option with Transmac, which I already tried. Unfortunately, I can't write to my Mac OS partition since it's read-only, which leads me to the problem of having to change it to read-write via single-user mode.

If anyone can help me, even in the smallest way, I would be eternally grateful and give you a big, fat kiss if I could. And I promise to stop tinkering with the system. Thank you!