.xlsx is the new 2007 file extension. Click on the logo, select Save As. Right across from it it says Excel 97 - 2003 Workbook. Resend to your friend.
As for the Virus - could be because you have attached a macro - but most likely his e-mail just didn't recognize the extension xlsx. Once you send to him in the format above he shouldn't have a problem.
Here's something extra - Microsoft has a free download for 2003 called the Microsoft Office Compatibility Pack. Here's the link: http://www.microsoft.com/downloads/details.aspx?FamilyId=941B3470-3AE9-4AEE-8F43-C6BB74CD1466&displaylang=en
Your friend can download the compatibility pack if you are sending things back and forth.
Here's two more useful tools for you. The first is a free Microsoft Excel training site. The first training session listed is Up to Speed with Excel 2007 - this gives you tons of information about the new layout, features etc. and is definitely worth the time. Here the link:
http://office.microsoft.com/en-ca/training/CR100479681033.aspx
The next link is an interactive Command Reference Guide.
http://office.microsoft.com/en-ca/excel/HA101491511033.aspx?pid=CH100648241033
Click on the "Start the Guide" button. You'll see the old version of Excel 2003 come up. Select a function (such as Data, Sort) it will show you where the command is located in Excel 2007.
I have Office 2007 and love it. The new layout and features take a little time to get used to but they do make sense.
If you need any other info drop me a line using Yahoo Answers e-mail.