well why don't you do what microsoft has done with XP.
Make them send the serial number through the internet to your website.
Then save it,(if it's correct) and if anyone else tries to use it it won't work.
And also like microsoft give them a certain amount of days to send it then delete or corrupt the file so they can't use it if it's not there in time.
you could do this by doing something like this:
firstly get the date
open to write 1,"something that people won't know what it is"+".chk"
write string 1,date$
close file 1
then do this:
make sure that the file is in the startup file in the startup file.
this wouldn't actually work but something like this
open to read 1,"something that people won't know what it is"+".chk"
read string 1,date$
close file 1
if date$ > date$+20
if serial#=0
delete file "multibillionsoftware"+".exe"
else
delete file "something that people won't know what it is"+".chk"
endif
endif
