XP re-install did not re-build registry !!!

Could anyone please explain this to me?

About a year ago I had to re-install Windows to get over a virus, and
even though it was overwriting the old o/s (the set-up process
confirmed this, as it gave me a warning), I noticed later it had not
built the registry from scratch. OK, the virus was sorted, but there
was a tell-tale entry for something that WASN'T standard Windows. It
was nothing important, and was something I myself had once added, but
I had fully expected it to be lost. Very odd, I thought at the time,
but didn't dwell too much on it.

Anyway..... one year on, and I'm again re-installing Windows, but on a
different PC. This time I completely removed and created a new (the
only) partition. Did the install business, yet once again, I happened
to find an entry which again proved it's not starting completely from
scratch!

How on earth?! I thought I had definitely vanquished all trace of
Windows first, this time. I mean, if you had a new PC wih no operating
system at all, and had to put Windows on for the first time, it HAS to
build a new regisitry from nothing then, so......

I'm confused!

Thanks
Max