Anything is recoverable if someone really wants to get it, However, clearing your cache should be okay. You can also make sure the Temporary Internet Files folder (for IE) and the cache folder (in the Firefox folders) are cleared out. Delete anything in them, then make sure you check your recycle bin and empty it.
Some computer tech have been know to scan through drives looking for freebies, including photo (yes, including porn) and music files. I've heard stories of guys copying stuff from customer drives for personal use. the clowns at that Big Box store (BB...) have been know to do this, but i wouldn't let them put new shoelaces on my sneakers.
You might want to post your problems here. Someone might have an answer that will prevent the need to visit a shop.