As we all know, deleting a file doesn't delete the file, it just deletes the file name from the... um... file handling system? Anyway, that porn collection, the lesson plans and your bank info are technically all still there on the hard drive. In due course they will be written over by the... um... file handling system? But while one is waiting on that, someone with some moderately sophisticated software can still get at your pics. To totally delete a file, you need a special program that writes blocks of nonsense data over the top of the file. I believe they are known as "shredders". (And if you genuinely do have top secret material to erase, the shredder needs to make something like seven passes before the file is completely gone.)
Personally, most times I've left a place, I just reformated the whole hard drive.