After your install fails, looks for a file called SEP_INST.log. Go to start > Run, type
%temp% and hit ENTER. If it's not in there, look in c:\windows\temp.
Wherever you find the file, open it and do a search for
return value 3. The 10 or so lines above that will tell you why the install is failing. If you like, paste those 10 lines in this thread and we'll take a look.