file assocation changed

Greets All,

For whatever reason my file associations have changed and I cannot restore via system restore.

In this case, I have a .pdf on my desktop which is trying to open with cmd prompt; knowing this is incorrect. I can however open the pdf using the 'open with' option on the context menu, but I'm looking to find out how to restore the association as it was before. I've tried using 'open with' and checking the 'choose default' method selecting 'Always use...' in the bottom left, but it's greyed out.

I haven't the slightest idea how I got myself into this predicament, so if you can provide any kind of feedback i'd appreciate it!

btw, I have no box available to return the pc to the retailer ;)