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