Has your minecraft ever crashed when you install modloader, Well there's an easy way to fix that
1: Type %appdata5 and go to your roaming folder and find a folder called .minecraft
2: Drag the .minecraft folder to your desktop
3: Start minecraft and type in your username and password and click remember me
4: Now Click Login and now your minecraft should show a froce update loading screen
5: if u had texture packs and world, just copy and paste them to your new saves and texture packs folders
6: Now try installing modloader again it should work now

hope i helped everyone