Although not totally XBOX related, but better to help than waste key strokes flaming (borrowed some of it from Windows help).
To change the program that opens a file
Open Windows Explorer.
Click the file that you want a different program to open.
On the File menu, click Properties.
Click the name of the program that you want to open the file, or click Other to select a program that is not listed.
Notes (or leave blank and it'll ask you which program you want to use or you can just delete the association)
go to Windows Explorer, then the Tools menu, then folder options, then click on the tab for file types and tweak up the associations.
To open Windows Explorer, click Start, point to Programs, point to Accessories, and then click Windows Explorer.
This change affects all files that have the same file name extension as the file you selected. For example, if you change the program that opens a .jpg file, all .jpg files will open in this program.
You can also specify a different program by selecting a file that you changed, clicking the File menu, clicking Open With, and then clicking a program name, or clicking Other, and then selecting the name of the program.