MikeKea--The wab.exe file is what makes the OE Address Book run and open. It normally resides in C:\Program Files\Outlook Express. As photolady has said, in WinXP the Address Book. itself, is in C:\Documents and Settings\UserName\Application Data\Microsoft\Address Book
You might consider restoring any corrupt files by Start|Run|type sfc /scannow|OK. You may be asked for your WinXP CD so have it ready.
Are you getting any error messages?
See http://support.microsoft.com/kb/917288/