I had been storing a copy of about 12GB worth of data, programs, etc. on my hard drive. My wife's harddrive was on it's last breath and would no longer load windows so I installed it as my "D" drive until she could get a new drive and her system was up and working. I used CDRWs to transfer the data. To be safe, I copied much of it to my "C" drive as a backup.

Now she's up and running. The "D" drive is safely stored away "just in case." I deleted the 12GB from my hard disc. It had been stored in one large folder with her name on it. How can I be sure I've really removed it? Is there some freeware or shareware that would be useful?

The program your looking for is Wipefile
If you have already deleted it from your hard drive
Then you will need to undelete it to use wipefile
or find a version of wipefile that will wipe free space

I use the Old version of this program from Symantec
but there are shareware programs that do the same thing..(by the same name)
One note about Norton/Symantec Wipefile
It will Not wipe disk free space in WinXP

At present the only thing deleting has done is to remove the link to get you to the data that you copied (like taking a label off from a cassette tape). There is still info on your drive but it will be written over as you use your harddrive more.

Most times people refer to the data as all 'zeros and ones'. As it works well in an explanation, I don't buy into it. It's an electrical circuit. All it knows is conductivity or nonconductivity. It is more like pluses (+) and minuses (-) or a yes or no for conductivity (letting electrical current pass through or not). Anyway, that's just my opinion or a way to look at it.

If there's sensitive information in the data from your wife's pc, then by all means use a wipe info software. But it is not necessary if there's nothing that needs to be held private. Just like tape recorders of old, the magnetic info can be recorded over or made anew just by writing to it again. As your 'C:' drive gets filled, it will be buried over and irretrievable or somewhat will no longer exist.
How can I be sure I've really removed it?You could try doing a search in your Start> Find> Files and folders box of any of the files that were in that folder or to look for that folder itself. If it comes back with either wrong answers (stuff from your drive instead) or empty, you've removed the "shortcut" effectively and the data will be covered over in time as you fill your harddrive space.