Error when starting MS Excel
 

I am using a networked computer (only 2 in the network) for a small business
office. Microsoft Office XP Professional was installed prior to my hire
here. We cannot find the installation CDs. Word, Access, PowerPoint &
Outlook all open correctly. However, whenever I try to open Excel, it starts
the Windows Installer and asks for the CD...I don't even know where to begin.
Please help!
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Thank you for your help!