Somehow you're file associations have gotten messed up and assigned a Microsoft Office program (probably Word or PowerPoint) to open PDF files.
If you really have Acrobat Reader installed, then do the following.
1. Make sure that you have downloaded the attachments.
2. Right-click one of the attachments.
3. Select the "Open With" menu item.
4. Select the "Choose Program" option
5. If Adobe Acrobat Reader is one of the program options listed, choose that.
6. IMPORTANT: Be sure to check the box that says "Always use the selected program to open this kind of file".
7. If Acrobat wasn't listed, then you'll need to click the "Browse" button and navigate your way to the Adobe Acrobat program executable. It will most likely be in
c:\Program Files\Adobe\Acrobat\Acroread.exe
For what it's worth, I've mostly given up on Acrobat and switched to Foxit Reader. It's smaller, faster and less likely to have it's security holes exploited. It's free. Google it.