hanslindgren,
If you write a file with .xls extension that looks something like this:
<HTML>
<HEAD>
<META HTTP-EQUIV=Content-Type CONTENT=application/vnd.ms-excel>
</HEAD>
<BODY>
<TABLE>
<TR>
<TD>Some cell</TD>
<TD>Another cell</TD>
</TR>
</TABLE>
</BODY>
</HTML>
When you open it Excel should display it as a a standard Excel document...