using computers will leave lots of stuff behind that you have to take care to delete completely.
1. as mentioned, deleting a file just moves it to the recycle bin [if you have that option turned on for the drive the file is on]. so, empty the recycle bin before you finish your computer/web activities.
2. as mentioned, an option is to shred the file. here's a nice one i've used for years
http://www.fileshredder.org ... after you install it, start it up, drag files onto it, then shred.
3. do you copy/paste text and/or images? then you need to clear out the clipboard. go into a text editor, Word, Powerpoint, and/or image editor, press Cntl-V or select the Paste function. there! that's what needs to be cleaned up. select all, type anything else, select all of the new text again, press Cntl-C or select the Copy function. you've just placed that on your clipboard. [some clipboards keep the x most recent copy or cut operations - watch out].
4. did you clean up your browser's history? if not, do so before you leave. or use the private browsing mode.
5. did you save an image from your browser to a folder on your computer? unless you have a diff option set, the browser will remember the last folder you saved to. guess what someone will see the next time they save an image? all those pics of pussies and blowjobs.
6. did you use an application to edit, view, save something? that document will be in your computer's Recent Items (most recently used or MRU) list. and those docs will show up in the apps most recently opened docs list. use something like CCleaner or Advanced System Care to remove stuff from the MRU list and other places.
CCleaner -
http://www.piriform.com/ccleaner
ASC -
http://www.iobit.com/en/advancedsystemcarefree.php is the free version; they have a paid one, too
be safe rather than sorry.