In a normal situation Microsoft Windows selects by the extension of a document which appliction it must start to open that document (.doc related to Microsoft Word, .pdf with Acrobat Reader and so on).

In some cases this goes wrong, has Windows lost his way and we have to help manually.

To look at the default programs, you select "Start" and chooses "this computer". Next you select "options" in the top of the screen and select "map options", the next screenshot will appear:

In this window you can select witch programm has to be used to open a certain kind of document. As an example, to open a .PDF document with the Adobe Acrobat Reader, you go to the upper part of the window and select the PDF extension by using the blue beam. After that, in the lower part of the window you select "change" and select the Acrobat Reader application as the default application to open this type of documents.

Of course, this works not only on the .PDF extension, it is possible with every extension that is known in Windows XP. And even if it is not known in Windows XP, you can add an extension yourself.