It's an Excel file, yes. This is a free viewer for that -
http://office.microsoft.com/download.../xlviewer.aspx
Is it a legitimate message though? Because it could be a virus. You're ok if you don't have Excel on your computer. Might be a good idea running a virus scan, unless it is a genuine e-mail you've been sent of course.
Virus scanner -
http://housecall.trendmicro.com/