data overwrite

i posted this question in the "lounge" forum over a week ago, and got no answers; perhaps someone in this forum can help me:

when CCleaner performs a "delete index.dat file" deletion under "run cleaner", does

CCleaner actually physically erase the data (by overwriting w/ fill or null characters),

or does it merely *mark* the disc blocks as deleted in the FAT/NTFS ?

tia (btw, i think this is a really neat product - kudos to the programmers/developers !

CCleaner doesn't secure erase the file, it just deletes it on the next reboot.