try force updating, and if that doesn't work go to your .minecraft directory and backup your saves, then delete every thing in the .minecraft folder and put your saves back in and launch minecarft. I hope that was helpful but if it still does that then there is an internal error or a conflicting external program that is doing it, in that case I cant help you.
If you download Winrar and you rightclick on the minecraft.jar file, there i an option to open with winrar, then drag the mod files in to that and your done