help overwriting free space

i was having a problem deleting the backup windows.old from the vista home
premium reboots. i changed the user rights and then i deleted the remaining
files successfully. heres the problem...

i have just put in PGP and its trial version so theres no tech support..
when trying to shred the free space, it says i need to run PGP as admin,
which isnt an option...thought it may be connected to the prob i was havin
not bein able to delete files cuz thats what it would say...need admin
privys. which i am on admin account...so shouldnt be a prob...
i know that when you empty recycle bin files are still there, so i was
thinkin i may need to go in and alter the rights of the deleted files??? then
i could overwrite the free space??? dont know too much about it at all, so
any help would be appreciated...