I have development experience (VS2005/SQL2005/ASP.NET), but I am new to customizing a DNN Module.
I would like to customize the Documents Module to highlight the Modified Date if the modified date is less then 14 days old.

In ASP.NET the code would be something like:

if ModDate > Today.AddDays(-14)  then
   response.write("<b>" & ModDate.toString & "</b>")
else
    response.write(ModDate.toString )
end if

How would I accomplish this customization to the Documents Module in DNN? 

Any point in the right direction would be greatly appreciated.

You can modify the Stored Procedure named "GetDocuments".  What I'd do is wrap the output in a span with a class identifier.  You can format with CSS.  I did something similar, which I've modified for your situation below:

Just use Sql Server Management Studio, modify the procedure, and replace "Documents.modifiedDate," with the below.  Then create a CSS entry for newDocument and updatedDocument.

    Case When Cast(Convert(varchar(20), Documents.modifieddate, 101) as smalldatetime)=Cast(Convert(varchar(20), Documents.createddate, 101) as smalldatetime) then
   Case When DateAdd(d, -5, GetDate())<Documents.ModifiedDate Then
    '<span class="newDococument">' + Cast(Documents.ModifiedDate As varchar(10)) +  '</span>'
   ELSE
Documents.ModifiedDate
   End
    ELSE
   Case When DateAdd(d, -5, GetDate())<Documents.ModifiedDate Then
    '<span class="updatedDococument">' + Cast(Documents.ModifiedDate As varchar(10)) +  '</span>'
   ELSE
Documents.ModifiedDate
   END
    End As ModifiedDate,

Good catch, Mitch is correct, i've tested, and an exception does generate.  I did not realize the front-end required a date.  What I did in my environment is added a new column so I wouldn't affect the existing code, so the original suggestion was the best route.

Sorry for any confusion.