1. Run Outlook as Administrator
Right click on your outlook shortcut, under the compatibility tab , check " Run this program as an administrator" (if you start to get popup's every time you go to open a program you can turn UAC off if your environment allows)
2. Check permissions on .PST archive file
Right click on the pst archive and goto the security tab, add the user "everyone" and give them full permissions and retry opening the archive.
3. Untick Read Only Attribute
Right click on your archive file go to properties and make sure that Read Only is unticked.
thats all folks, if your still having issues. You may be trying to access your pst from a network location which is not recommended by Microsoft but theoretically should still work if not , copy it across to a local directory and try again.