I've been experimenting with mods for Minecraft recently, and started by downloading Buildcraft, including Modloader, ModloaderMP, and MCForge. The game worked fine. I had fun. Then I installed Equivalent Exchange and Recipe Book. Much fun ensued. Recently, however, I installed IC, minus the sound fix, and it worked fine.
When I wanted to show my friend my mods, I wanted to show him a world from before I installed IC, so I went into the config file and turned the value for IC from "on" to "off". Later, when I wanted to turn the mod back on, I changed the value back from "off" to "on". Then the game would freeze at the Mojang screen, and I would have to force close the program.
I noticed that if I went into the config file and changed the value from "on" to "off" again, the game runs fine with the other mods. However, I backed everything up and created a new file and reinstalled everything just to be safe. It still freezes at the mojang screen. After this, I got concerned, so I only installed IC, including all necessary mods (modloader, modloadermp, crashfix, forge, etc.), and did not reinstall the other mods. Once again, it froze at the mojang screen. If I change the value in the config file to "off" or remove the IC jar file from the mod folder, the game runs fine. Therefore, it is not a compatibility issue.
It is important to note, however, that IC did in fact run perfectly fine until I disabled it in the config file. After that, it would not load, and reinstalling does not fix this. I suspect that the Jar file might be incomplete or corrupt now, seeing as others appear to have similar problems, but the problem may also be a problem with Java itself, which is the only program I did not reinstall. I hope all this information helps, and I hope this problem is fixed soon.