When windows deletes soemthing, it simply marks the space on the hd as overwritable.
To truly delete something, the space where the file to be deleted occupies need to be written over with random strings of charactors. each time the space is written over is called a pass. it takes about 4 passes for it to be truly gone, and veen then determined investigators with $20000 hardware and software can still read it. 8 passes will render any software data anaylsis tool useless, but still be read with special bomputers. At 16 passes, that file is gone. Only a determined team of FBI agents or similar with government mainframes can get to it.
each pass uses a different kind of standard. The two most popular are:
-Guttman standard, which requires that the file be overwritten 35 times with pseudorandom data.
-The other is published by the Department of Defense, Manual 5220.22 M. This requires that the file be overwritten with 0's, then 1's, and then random data; and further stipulates that, for top secret data, the disk on which the file is written must be deguassed, and then destroyed.
Generally, the guttman standard is used. (Most people can't afford to conform to the DOD standard). There are many softwares that claim to us this method, but be careful. I highly recommend pgp personal which comes with pgpdisk which can perform data wipes and are highly effective at truly deleteing data. PGPdisk even has a option of free disk space wipe, which will destroy all data that has been deleted before (i.e. simply marked for overwrite). This is a good application for you as you can clean out your temp internet folder with disk cleanup (a windows utlity) and then do a free space wipe. PGPdisk also supports up to 28 passes which is insanely good.
You can buy PGP here:
http://www.pgp.com/products/desktop/personal/index.htmlA great buy.