I agree with CCleaner. I use this program every night after a full day's worth of browser usage, downloading files, deleting files, etc. You can manually set the overwrite limit (I believe up to 35 pass) and select between a good number of options of things to clean. It will detect with programs you have installed that are "Cleanable" (IE, Opera, Firefox, Winrar, regedit, clipboard, MS Paint, etc.) as well as the recycle bin and temporary files. I have found the cleaner to be quite effective. Also, it can do a registry cleaner that works pretty well. It is cautioned not to clean the registry simply because it can be easily messed up, but I have had no problems the 6 months that I've had the program. It also has an uninstall list similar to Add/Remove Programs list as well as a start-up list of programs that start with your computer (very useful if you do not know how to check it in msconfig).
You can add Ccleaner to the Right-click context menu, have it set to run on start-up (though I don't since it does not inherently have an "always-on" cleaner, only on-demand), and have it auto update. It doesn't use a lot fo resources, and it loads very quickly. I would definitely recommend it for everyday tasks. There are other programs out there that are more intense and secure, but this is an excellent freeware program that I personally use all the time.
Check out Piriform, the company which makes CCleaner. They have other programs that I use, as well, that might be useful to you: Recuva (File recovering program for deleted files) and Defraggler (Defragging program that seems to be more thorough than the WinXP one).http://www.piriform.com/