Vista - Personal Folder not closing properly

I don't know if this is the appropriate forum to post this problem, but as it only started happening after I upgraded from XP Prof to Vista Home Premium I presume it's a Vista issue.

Recently I've been getting the following message when opening Outlook:

"The data file 'Personal Folders' was not closed properly. This file is being checked for problems"

I then have to wait several minutes while the file is checked before I can start using Outlook.

It doesn't happen everytime I start Outlook, but often enough to be irritating.

I have tried closing Outlook using the "Exit" option on the "File" menu and also the "X" option, but it doesn't seem to matter which method I use. The "not closed properly" message seems to appear at random.

I'm using MS Office Outlook 2003 SP3 and running Windows Vista Home Premium.

I would welcome suggestions on how I can fix this issue. Thanks.