There is probably something wrong with the minecraft folder. If the error message prevents you from playing, you should move your saves folder to another location and delete the minecraft folder. Then open up minecraft and it will make a new folder. Then put your saves in the new folder, and it should work.
Simply drag you old world maps from your old "saves" folder to the new "saves" folder. Application Support/ minecraft (or .minecraft for Windows) /Saves.
A folder called .minecraft
The saves folder is in a hidden folder in application data called .minecraft. The directory to the folder is: My documents/Application Data/.minecraft. There is a folder in .minecraft called saves which contains your saved worlds. If this folder does not exist, then perhaps you have not yet played any worlds on that computer or somebody has deleted the folder.
It could. If you want to save it then go to the save folder and make a duplicate or drag it into another folder and move it to your desktop, Documents or whatever folder.
you have to go to start, click run, click your .minecraft folder, take the downloaded mod (it has to be zipped) click open archive, then with the .minecraft folder, you find bin go in, and open minecraft's archive, then you move the files in the mod archive folder to the minecraft archives. (some things might be wrong, but that's because i used pure memory to answer this, and i have a bad memory. )
They go to the screenshots folder in the .minecraft folder. Click "Start" and search for "Run". Type in %appdata% and press enter. Find the .minecraft folder and look for the screenshots folder.
The minecraft.jar folder ( in the bin folder, in the .minecraft folder, in the application data folder) note, you will need to use an extracting program like winzip to open the folder
Locate your Minecraft folder, then find your save file. Copy and paste this onto your hard drive. Minecraft is installed in your AppData folder (C:\Users\[username]\AppData\.minecraft\) The saves folder should be in the .minecraft folder.
I can give you half of the answer: My friend told me that Minecraft somehow doesn't allow you to move Minecraft on a USB Drive and play it on another computer. I even tried it, and it doesn't work.
When you open minecraft.exe, the folder is automaticly created. To access it, press start --> run and type "%appdata%". then you should see the folder .minecraft. That's the folder.
As minecraft looks for "minecraft.jar" in your "application data" folder, it is impossible to "move" or "copy" any file from there into a new location and run it from there. There has however been a "minecraft portable" mod, which allows for movement of the .minecraft folder.