Before I show you how, know that this is why having a disk drive next to your computer is the best way to get your computer hacked. Applies irl too, you leave your computer unguarded and it's too easy for someone to insert a flash drive that autoruns a virus on it
On the majority of passworded computers, there is no security installed against disk boots that can override the computer startup, or worse. Here are the quick steps to crack those computers that don't have such protection and also have a disk drive next to it (or you can be sneaky and add your own)
First, insert your floppy onto your own disk drive, and in your computer, type the following:
1) cd disk
2) edit startup
3) print("Access Granted") --Or any valid command, really
This pretty much edits your disk's startup to whatever you'd like. Press Ctrl+S, then Ctrl, right arrow, and enter to save that to your disk. Then, insert your disk into the target computer's disk drive. Because disk startups have priority over computer startups, if you boot the target computer, it will run your disk's, not its own. And if it's already on, just hold Ctrl+R to forcibly reboot it.
I had maybe 30 minutes of play time with Tekkit total when I posted this, and this just goes to show how common this knowledge is. Other LUA programs could just as easily delete your startup program, or totally replace it with a program that sleep(99999999)'s your computer with program termination during runtime disabled, with password protection against disk boots. In other words, completely locks your computer up for a long time. I don't know if people store stuff on these computers, but I can't imagine having to break and replace your computer a good thing.
Moral: Don't have disk drives or any way for uninvited "guests" to place them next to your computers if you operate a password door with it. Oh and, relating to door security, have a player sized hallway leading to the door made of obsidian if on a pvp raiding server. There's more conventional ways of gaining access through an iron door, and it involves explosives
I might look into ways of cracking one without a disk drive and it'd probably go along the lines of code injection or something. If I figure how, I'll post another tutorial. kcya.