Outlook 2007: Capacity, Archive, and Backup

I run Office 2007. In the past few days I have been receiving a new (to me) message from Outlook. It basically says that Outlook has reached its capacity. I save almost all email, so no wonder.

If I use Outlook's archive feature I think I can free capacity so that Outlook returns to working normally. But how does this affect backup? I normally backup by copying my pst folder to an external harddrive. Would this process also backup the archive?

Thanks.

you are probably using outlook in caching mode.
the .pst file usually won't report the message, it is your exchange server.

for .pst file size see here
http://support.microsoft.com/kb/830336

to turn of caching mode and save the emails to your .pst only
in outlook go Tools > Account Settings...
double click your exchange account
take check out of Use Cached Exchange Mode

This will download the messages to your pst and remove them from the exchange server.
Otherwise increase your message store size on your exchange server.

If it is actually due to .pst size you will probably want to backup both your .pst file and your archive.pst file to save all your emails