See if this file association fix from Doug Knox will cure your problem:
Out of the blue my computer opens everything in Corel Photo House and pops up with invalid format. The only thing I can access is Internet connection. Even Restore opens up in Corel. I cannot uninstall Corel as it does not show up in add/remove and when I go to uninstall in Corel's program it also comes up invalid format. I am about to reinstall Windows xp but would love to avoid that, if possible.
Need help. I have not installed anything new, checked for viruses, none, etc.
Running Windows XP.