and it's appearing as a result of the April 2003 Cumulative Patch for Outlook Express (330994). The patch is installed for Outlook Express 5.5 or 6 in response to a vulnerability that could allow an attacker to run code of the attacker’s choice on your computer. The problem is, there was a bug in the patch.
Whenever you make a change in your Windows Address Book file, Windows makes a backup. Usually this backup is called username.wa~ , however after the patch is installed the backup gets renamed to just ~ instead and saved in the directory where you start your Outlook Express. Most people start OE from a shortcut on their desktop, so the backup file gets placed there.
As VoG™ says, you can safely delete it, although as a precaution you might like to rename it with a .wab extension - say: addressbook.wab and send it to a floppy or some other removable media. Then if a disaster strikes and your address book vanishes you'll have a backup.
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