I've given away a few dozen PC's in the last 10-plus years, but never
to anyone I know. I agree with the folks who recommend wiping the
drive. Formatting or repartitioning aren't good enough, IMO. Data can
easily be recovered after those operations.
Several points:
1. Although it's easy to recover data after reformatting, not everyone
knows how.
2. Not everyone has any interest in what someone else had on their
drive, and many (probably most) people wouldn't bother to even look at
it. I, for example (who knows how and have gotten several used
computers and drives), have never bothered to even look at them,
whether or not they had been formatted or anything else.
3. Formatting followed by reinstalling Windows makes it much more
difficult (but not necessarily impossible) to recover data.
4. Depending on what was on the drive,not everyone cares whether
someone else sees it. For example, someone who uses his computer for
nothing but playing games probably doesn't care at all.
So what you should do depends on what's on the drive and how paranoid
you are about it.