Answer To Mod Problem
--------------------------------------------------------------------------------------------------
1. Open Minecraft (Don't Login)
2. Click The Options Button (Right About Login)
3. Click Force Update
4. Click Done
5. Login
6. Your minecraft.jar is now clean
You didn't lose any texture packs or worlds, don't worry
PS: Your Minecraft Might be crashing maybe because of the following list.
--------------------------------------------------------------------------------------------------
1. Not Enough RAM
2. Older PC
3. Virus
Hope This Helps