right - if you delete a file even from the recycle bin the information isnt removed from the harddrive. it is just made less accessible. However it is very very hard to access it. so if you did remove it the new owner shouldnt have been able to retrieve it easily and if she did it leaves questions about why.
In order to completely rmove stuff you need an overwritter because even reformating doesnt remove all of the digital information. Police specialists can retrieve information from reformated harddrive. The overwritter is like a shredder that you can put on to run but doesnt remove the stuff you do want (i think) but clears the rubbish. There are several free ones. One reported to be used by the FIB is supposed to be good.
However the only failsafe way of clearing a harddrive is a giant electromagnet of doom.