MORE INFORMATION
http://support.microsoft.com/?kbid=287623
Use the steps in the following section to reset the Outlook nickname cache for Microsoft Windows XP.
1. Quit Outlook.
2. Start Microsoft Windows Explorer.
3. On the Tools menu, click Folder Options, and then click the View tab.
4. Under Advanced Settings, click to select the Show hidden files and folders check box.
5. Click OK.
6. Click Start, and then point to Search.
7. In the Search Companion box, click All files or folders.
8. In the All or part of the file name box: box, type *.NK2.
9. In the Look In box, click to select your local hard disk.
10. Click Search.
11. Right-click the .NK2 file with the name of the profile that you want to reset, and then click Rename.
12. Rename the file to profilename.bak, and then press ENTER.
13. Quit Windows Explorer.
14. Restart Outlook.
Once the *.NK2 file is located, set it up in CCleaner - "Options/Custom" - to delete the file when run. Outlook will regenerate a new "blank" one.