You can create the most basic of init.ora files to get your instance up and running, eg

For example, something like this:

control_files=("control01.ctl", "control02.ctl")
db_block_size=8192
db_name=mydb
diagnostic_dest=c:\oracle
undo_tablespace=UNDOTBS1

would probably be enough to get you up and running, as long as you know where your controlfiles are. If you look in your alert log from the last time your started up, it normally will list all of the important parameters that were used to start the instance, so this gives a good starting point.

If you dont have even that, you'd need to have an RMAN backup of your controlfile, because then you could do something like:

startup nomount

which just initiates the memory structures and then restore the controlfile from an RMAN backup.

Good luck !