Which mail client or mail service are you using? Usually when opening an attachment, a temp file of the document is created and opened. The folder the temp file is being created in may not have permission to write there. I'm not sure how that would happen as each user profile should also have it's own temp folder that is read/write. Find your temp folder. I beleive it's under c:\users\<your account> somewhere and reset the permissions on it.
Also, you may want to reinstall adobe reader to make sure file type registrations got added properly.