As he said - the trick is to ask yourself just how secure you wish to be, since there are many things you can try. You could...
- Replace the HD - since you're in need of keeping your data, there's no rule stating that you couldn't simply add the old HD to the new computer as a second HD and simply install a new HD into the old machine - the only problem would be that you would have to install a new OS into the old machine.
- Hit the RESTORE command on the old PC after saving what you wanted to. Depending on the make and model of this old PC, if it is an old 'package' machine (a pre-made Dell, HP, Compaq, eMachines, etc...) that was purchased after 2001, many have a simple HEAVY RESTORE command (for example, in HP desktop computers, holding the F10 key just after turning it on will activate this function). When this is run, the computer will restore itself to the way it was the day it was turned on, and all data is erased (including any added programs and hardware add-ons).
In either of these cases, you will be wiping the data, but oddly, the data can still be accessed by someone with the proper software to read this type of deleted material. This means that there are only two other options that would be safe -
First - again, simply install a new HD in the old machine and reinstall the OS. Your old HD becomes a paperweight on your desk, or you can install it into the new machine (as stated above) as a second HD.
Second - get a 'Zeroing Program' - this program (free from Seagate/Maxtor) litterally starts on one end of the HD and writes 0s across the entire drive (depending on the size of the drive, this can actually take HOURS to do). This not only clears the old memory, it also removes any bad sectors of data that were labeled BAD simply because of corrupt data (it will not fix bad sectors of the HD if it is a physical/damaged secton of the HD).
Eraser is a good program - I prefer MaxBlast from Seagate/Maxtor.
Good luck!
A-N