I can't seem to put my hands on it right this second, but there is a base64.exe floating around here/the net. You can also use the UDF here on the board...

base64 - see last post for an updated version
http://www.kixtart.org/forums/ubbthreads.php?ubb=showflat&Number=119906#Post119906

How to use UDFs -
http://www.kixtart.org/forums/ubbthreads.php?ubb=showflat&Number=81943#Post81943

The rest of the UDFs are here -
http://www.kixtart.org/forums/ubbthreads.php?ubb=postlist&Board=7&page=1