1 From within outlook select file option then expand cleanup tools and select archive
2 Make sure you select 1 year to start with and select browse to choose a location from the root of c drive (create a folder with your initials and change the archive.pst to archive1.pst if one already exists)
3 Click okay and it will archive your data releasing storage space
4 The archive will show as a separate folder on the left hand side and can be searched by selecting all mailboxes if required
5 Repeat option 2 by choosing another date ie 9 months which will free up more space