How to really "clean" one's PC? (Windows 7)

Hi everybody.

For my job, I use my computer (Windows 7 Professional) to produce sensible data about projects, sales, etc. To do that, I use several kinds of programs (MS Words and Excel, Adobe Photoshop and Flash, Autocad, statistical analysis software, and some others). After the work, I save all the files on a pendrive and (try to) remove all the "accessory" files created by the programs (application data, Temp folders, etc.). I also use a program (I don't remember its name now) that should definitely remove deleted files. However, I can't be sure that really my machine has been cleaned, and there is no trace of my work... what if I forget an accessory file? This has become a main concern for me because I suspect that someone in the office is able to access my machine (even if, of course, I set a password).

I didn't find the right solution from the internet.
References:
Networking/Security Forums :: View topic - How to really "clean" one's PC? (Windows 7)

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