my computer and then on Tools. Select Folder options from the menu, and then click on the tab marked 'File types'.
Now scroll down until you see PDF - it will have Photoshop listed as the default application for opening this type of file. Click once to highlight it and select the 'Change' button - now select Acrobat as the default application and you should have no further problem.
As a general rule it's a good idea to watch carefully when installing new applications - many of them indicate that they are going to set themselves as the default for certain file types, and you have the option to prevent it if you want to.