Hi Quentin,
This could be being caused for several reasons. The most common is that the file was not created using Excel, rather SAP or some other program that can create Office documents. Often these documents are not exact Office documents and have different header and tail information than the file created within office. To test this, simply open the file in Excel and resave it under a new filename and attempt to upload this file. If this file then uploads fine, the problem lies with the application creating the documents. Normally it is the "Scan Doc files as containers" general option that triggers these detections.
If this is not the case, then it would be useful to see which anti virus engine is making the false positive detection. This can be seen in the Programlog.txt log file by opening it following an attempted upload and then searching from the bottom up for the filename. The file will then need to be submitted for further analysis. The following KB describes how to do this -
http://support.microsoft.com/kb/952163 I hope this helps,
Alex