You do have the applicable software installed, correct?
You fail to state if this worked before now.
Two Options:
- IF you DO have the software installed, remove it via Add/Remove Programs, then reinstall
- Open My Computer, Tools menu, Folder Options, File Type tab, look for PES in the Extensions column
If you DO have the software installed, and PES is listed or not, you can change/add a file-type (aka extension), see screenshot with PDF example.