OE6 compacting messages nag box

This is probably an ez question for someone to answer, but I haven't found it myself so here goes:

When I use OE6 and then when finished with all my emails, then close OE6, a nag box immediately pops up that reads:

To free up disk space, Outlook Express can compact messages. This may take up to a few minutes. OK or Cancel.

I always hit Cancel cuz I don't want to compact any messages. I've been all thru Tools>Options and can't find a button to turn this off. Annoying as all get out.

Thanks in advance for any help with this.

HP a1412n
Intel Viiv w/dual core processors
200GB hard drive
512MB RAM
DVD+-RW Lightscribe Double Layer
IE7
Windows Defender
all MS updates current