error can't find project or library

I have some vba Excel code that works when it is run on a windows 2000 OS PC But when I run the same code on an Windows XP OS PC I
get
Compile error can't find project or library

both PC have Office 2003 (Excel) installed

Debug brings me to

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If I get rid of the Format function it solves the error

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If I get rid of the Trim function it solves the error



How can I resolve this issue so that my VBA code works on both Windows 2000 and XP OS???

Tools>refernces

it should have a checkbox and prefixing it should say 'MISSING'