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When you-
"sell or give away a Windows machine, I would never do it without first wiping the hard drive, thereby solving any concerns about it being "gummed up" with viruses and spyware"
...just how did you "wipe" it? XP/DOS delete/erase? Format maybe?? Or even FDisk to eliminate a partition??? Guess what, your data is still there! As is all the adware/malware/etc and even a noob could restore most of it (including the porn, of course) unless the drive saw serious write activity subsequent to your "wipe".
Now, if you actually used government spec erase software that actually WRITES OVER your old data, you may be safe. Did you do this? If not, you really didn't accomplish jack with your "wipe".
Mac users needn't waste time and money looking for software to do this- "Secure Erase" technology has been built into OS/X for ages.
~gtd