To add a little to what Cal says:
When the processor needs to write to the hard drive it uses a
F.A.T.(File Allocation Table) cluster for each writing..The Standard FAT size is 512 Kbytes..
If the program writing to the disk is 1 MB it will use Two FAT Clusters of the size 512Kbytes......A 10MB File would use 20 clusters etc...
So you use the program then turn it off or delete it...
If the next program to grab these two FAT clusters is a program Size700Kbytes then there is still 300Kbytes(1MB-700Kb=300Kb)
of the first program on the second cluster...(+ slack space)
This 300Kb of program or File can be retrieved from the disk even though there is no sign of it seen in Windows or DOS...
If you have Wiped the File/Program(overwrite the stored bits with 1s' and 0s')
then Nothing can be retrieved ...
The first time I wiped my slack space it freed up over a Gigabyte of drive space..
NOTE: Extreme caution must be used with "Wiping Utilities"..Wiped files no longer exist ,to any large degree, and cannot be brought back...
When you merely Delete a file/program to the Recycle-bin, you only cut the string(link) to pull it out..The File/program remains on the disk till it is overwritten by another program or the FAT cluster is WIPED
by a Wiping Utility such as the one in
"ACE Utilities"
jazz.........