With Service Pack 2, Outlook Express is set to ask about Compacting messages after every 100th closing of Outlook Express. Before Service Pack 2, the default behavior had Outlook Express set to compact messages in the background which has lead to the corruption of the message store. Compact the messages manually, reset the Registry value which keeps track how many times Outlook Express has been closed.

Look for this key in the Registry: (run regedit by going to start > run > type in “cmd”)

HKEY_CURRENT_USERIdentities{GUID}SoftwareMicrosoftOutlook Express5.0
Double click the Compact Check Count
Select the Base option for Decimal and change the value to 0
This should change both the Hexadecimal and decimal values to zero.
Close the Registry.

Reference: http://forums.techguy.org/business-applications/479919-solved-free-up-disk-space.html