To make Adobe Acrobat X the default viewer for viewing PDF files, follow these steps:
1. Right-Click on any pdf file.
2. From the menu that appears, Select Open With -> Choose Program
3. Now browse and select Adobe Acrobat X from the available programs
4. Check the Checkbox present at the bottom of the list that says "Always use the selected program to open this kind of file" and press OK.
The default PDF viewer should change for you. If you need further assistance in this regard, browser to http://support.microsoft.com/kb/307859.
Do let me know if this information helped you.