What is a .xll and should it be checked for virsusses?

Ballistics for Excel
An .xll file is a regular .dll file containing a special interface for Excel. A .dll file is a Dynamic Link Library containing executable code. It is a good idea to have the bfx.xll file and the Excel workbooks from this site checked by a virus scanner. I take great care myself to keep everything virus free, nevertheless it is better to be safe than sorry.