I thin this is what your after. before you do it, 1) make sure you have a backup of all your emails and 2) do a search for archive.pst on ur PC as you will need to specify where this is below (this is what the archive file is normally called)
On the File menu, click Import and Export.
Click Import from another program or file. Click Next.
Click Personal Folder File (.pst). Click Next.
In the File to import box, type in the path to archive.pst file (use the browse button is better!). Click Next.
Click the folder to import from. If you want, click Include subfolders.
Specify how you want Outlook to handle duplicate items.
Click Import items into the same folder in, and then click the folders with the same name as the folders you're importing from.
Click Finish.
hope that helps!