Open the door first of all...
Joking apart, just deleting your files does not make them invisible! They can be restored very easily. If you do not have any "secret" information in there or vital data, then what are you worried about?
I can also confirm that if there is a chance for someone to have a look at your old data in your hard drive they will. I have seen it, lived it and read about it pretty much everywhere.
If on the other hand you do not want them to find anything, the simplest solution is to do this:http://www.dban.org/http://www.zdnet.com/blog/storage/how-to-really-erase-a-hard-drive/129
There are many programs, others free, others you need to pay for, that will securely erase your hard drive. But that takes time since the programs need to access the whole hard drive, probably more than once.
I know this information is a bit late, but keep it in mind for the future.