If private files were deleted from the computer a long time ago then if the computer has had a lot of disk use since then thwy might have been overwritten by something else already, they might however still be there in the free space on the disc and be recoverable unless fotware is used to overwrite the free space on the disc. 1 pass of overwriting should be enough. CCleaner can overwrite free space like this(P.S. CCleaner is made by piriform).
If you have other private files which have not been deleted yet then you could save yourself the trouble of needing to run free space overwrites for them by using a secure deletion tool for them. If you have AVG as your antivirus it has such a tool built in, there are also many other programs which either exist solely to securely delete files or contan functions to do so alongside their main uses. These all work the same way in principle, overwrite the file with junk data so it cannot be recovered.
As for files on the USB, things are a bit different here, when you delete a file from a USB the file is usually actually erased, unlike files deleted from HDD hard drives (where the file's location is just forgotten but the file remains until overwritten). To be sure you might want to format the USB (which will erase ALL the data on it, so first copy off of it anything you want to keep), or if it is a really cheap USB you might just want to submerge it in water for a while then hit it repeatedly with a hammer, making sure to have copied from it onto other devices any files you value. If you choose to smash the USB into oblivion it is probably best to format it first anyway, just in case.
Edited by rp88, 15 June 2015 - 12:25 PM.
Back on this site, for a while anyway, been so busy the last year.
My systems:2 laptops, intel i3 processors, windows 8.1 installed on the hard-drive and linux mint 17.3 MATE installed to USB