Are you using outlook, or outlook express?
If outlook express, there are some dbx files that express uses, they are hidden, to find and rename the file(s) follow these steps.
1.......open 'my computer'
2.......click on 'Tools->Folder options'
3.......click on the 'view' tab
4.......place a check mark in 'show hidden files and folders'
5.......click 'ok'
6.......click or double click (depending on your setup) on C drive to open the root folder
7.......open 'documents and settings'
8.......if the person has an account in xp, the name will appear here, open it.
9.......open 'local settings'
10......open 'application data'
11......open 'identities' if there's more than 1 folder, follow steps 12-15 until the previously renamed file is found
12......open a folder
13......open 'microsoft'
14......open 'outlook express'
15......look for the previously renamed file, then rename it to 'Inbox' (exactly as typed).
16......follow steps 2,3 and reverse step 4
17......close open window.
The data inside the file will still be intact.
start outlook.
Hopefully this should help, bearing in mind, this is for outlook express, I'm not sure if it's the same for outlook, maybe someone could find out.