You can create pfile if your database was started up by spfile:

create pfile from spfile;

After you have a pfile, you can edit it and modify as
you wish and startup your database:
startup pfile=...

Of course, you can create spfile too:
create spfile from pfile;

You can also modify parameter stored in spfile by:
alter system set xxx=yyy scope=spfile;

On window 2K, you can edit most
of parameters directly though it is not suuported by
Oracle because spfile is binary.