Found it here while looking for info on Compacting,Turning off Automatic and Canceling.

http://www.microsoft.com/windows/IE/community/columns/filecorruption.mspx

As stated earlier, compacting all folders in Outlook Express frequently repairs mild corruption. But surprisingly enough, and again as ironic as it seems, the compaction process itself has also been implicated in causing corruption. Before Windows XP Service Pack 2 (SP2), Outlook Express was configured by default to compact files automatically after Outlook Express had been inactive for some minutes. However, although Outlook Express might have been inactive, the same could not always be said for other programs being used when compaction began. On some systems, particularly those with limited amounts of RAM or slow processors, the sudden drain on computer resources sometimes caused the compaction process to hiccough, and all too often a damaged DBX was the result.

It is largely for this reason that the background compaction feature has been removed in Windows XP SP2. Now Outlook Express will compact automatically only after you have opened and closed Outlook Express 100 times. It will do so not in the background, when you might be busy doing other things on the computer, but rather when you close Outlook Express for that 100th time. Do not cancel this compaction, nor use your computer until it is finished. For best results however, you should still compact all folders on a regular basis, such as weekly or bi-weekly.

If you have not installed Windows XP SP2, you should disable the background compaction to lessen the chance of it damaging your files. To do so:

1.

Click Options on the Outlook Express Tools menu.

2.

Click the Maintenance tab.

3.

Clear the checkbox for "Compact messages automatically in the background".

4.

Click OK.