If you have located the .pst file that Outlook was previously using this article includes intructions for importing the data back into Outlook:
http://support.microsoft.com/default.aspx?scid=kb;en-us;287070
Unfortunately, sometimes there is data loss when performing a 'Refresh' install of Windows XP (In-Place Upgrade). Data should really be backed up before performing the process of re-installation, for safety's sake. I hope the .pst file you've found is the one you need.
Catweazle
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