Thanks to everyone, I have solved the problem.
I succeeded by installing the package "latex2html", which includes the file "html.sty".
NOTE THAT THE FILE "html.sty" LISTED IN A PREVIOUS MAIL IS NOT VALID BECAUSE I GOT THE
FILE "typehtml.sty" FROM ANOTHER SYSTEM AND RENAMED IT TO "html.sty" TO CHECK IF THE
COMPILATION PROBLEM DESAPPEARED. BUT IT DIDN'T WORK. APOLOGIES FOR THE ERROR.
Thanks,
Manuel Arenaz