Cannot open any files on pc I just get notepad with unreadable message can only read cannot open file in dos

your file associations have been corrupted.
If a program does not show up in the list, you can make the program a default by using Set Associations.
1. Open Default Programs by clicking the Start button, and then clicking Default Programs.
2. Click Associate a file type or protocol with a program.
3. Click the file type or protocol that you want the program to act as the default for.
4. Click Change program.
5. Click the program that you want to use as the default for the file type you selected, or click the arrow next to
Other Programs to show additional programs. (If you don't see Other Programs, or your program
is not listed,
click Browse to find the program you want to use, and then click Open
6. Click OK
 
right click on an avi file and change its default program by choosing open with from the little menu.
a box will open.
you will see a few files listed as the defaults.
make sure the use as default check box has a tick in it.
then select the appropriate executable/app to open. then click on ok
do the same for all known filetypes.
avi's for instance - vlc/windows media player.
jpeg is associated with pain or photoshop or what ever.