|"There is only a new Outlook.pst file in my user file started 28 July 2011" That's probably the one that you're currently using. How large is the file? Have you searched for pst files on your hdd?|
"I am not sure which program to use to open it" pst files are used with Outlook. If you have more than one file that you want to use, then you would go to File, Open, Outlook Data file, and browse to the location of the pst file. To see which pst files Outlook is currently using, Click Tools, Account Settings, Data Files.
"I have noticed that even though I have lost the address book, the full addressee's address appears in a drop down box as soon as I type the first letter. I am not sure if this is a clue or not." That's the auto-complete/suggest names feature, which is not related to any pst files, but rather your Outlook profile. These names are stored in the NK2 file in your Windows Profile.
When was the last time Outlook worked properly, and what changed between then and the point when you restarted and could not access your old mail folders?