archive.pst is always being amended

Hello

I'm running OL2002 under WinXP as an e-mail only
configuration. The auto archive function on each folder
is set to off.
When I start OL and run the first send/receive, the
archive.pst file is always amended, though I can't see
why. Subsequent send/receives may or may not amend the
archive - sadly I can't detect any pattern here.
On the face of it, this is no big deal, but it's a real
pain when I back up to my portable PC (which I must at
least 3 times a day). As my archive file has seven years
of data and it's just over 400Mb, you may begin to see my
frustration!
If anyone has a solution to this, I'd be very grateful.

Peter Palmer
(Normandy, Surrey, UK)