You pressed Adobe by mistake. Ok, no biggie, things happen. Did Adobe ''read'' the file? Did you ask Adobe to save the file back to the disk? If so, that presents a problem. Adobe messed with it. If you did not ask Adobe to save the file after opening it, the previous file is unchanged.
This file association business is a separate function within the operating system. When a program is installed (such as Adobe), it tells the O/S that any file having a .pdf (and a few other specific) extension should be shown with a red flying ''A'' icon and opened by default by Adobe. When Word is installed, it tells the O/S that any file having a .doc (and a few other specific) extension should be shown with a blue ''W'' icon and opened by default by Word. When Excel is installed, it tells the O/S that any file having an .xls (and a few other specific) extension should be shown with a green ''X'' icon and by default opened with Excel. But the document files are unchanged. However, if the program continues to use the same filename extension through several versions, they all look the same to the operating system. Xls is still Excel. But if it happens to be a newer version file, the older version program can't deal with it. Also when there is improper usage of a filename extension (some other program created a file and used the same extension, or a filename was manually changed), the operating system will still try to use the default program to open it, but the program will fail. If it should occur that a Windows is presented with a file having an extension that no program yet installed on that machine has claimed as its own, Windows will stop and ask you what to do, giving you the list of available programs. If you choose one, it will likely assume your answer as a permanent choice, and construct a new association from then on.
Yup, WindowsXP is pretty smart and can repair most of its own mistakes, but it's not that smart. It usually can't repair ''mistakes'' inside files that other programs have caused.