I have been using Microsofts Outlook backup utility for a few years now
with good results.
http://www.microsoft.com/downloads/details.aspx?familyid=8b081f3a-b7d...
It will schedule your daily backups to where ever you define your data
to backup to, each day you shutdown Outlook.
I then use Acronis True Image 9.0 to backup my data files daily, as
well as an image of my drive.
Your Outlook.pst file is your active Outlook working data. The
archive.pst file is
really just that, an archive of old static data that you want to keep
separately from the current active data. I only use and backup the
Outlook.pst file, but that is personal choice. I'm sure some people
maintain
an archive.pst file too.