pick up a penguin
Corporate-license Windows, so we can't give that away, nor most of the other software that's installed. So nuke them to Linux. Easy-peasy if your O/S wants to be copied.
boot a stand-alone linux disk
overwrite the whole disk with zeros - we don't have any really sensitive data, so this suffices.
# dd if=/dev/zero of=/dev/sda
Google DBAN if you need a more secure erase.
install a free linux system image
# mkdir x
# mount server:/images ./x
# cat x/linux.img.gz | gzip -d -c | dd of=/dev/sda bs=1M
shutdown, reboot, redeploy as an X-terminal, casual browser terminal, or give the system away. (Get a signed acknowlegement, just in case the new owner dumps it and the PC is traced back to us). They'd also be perfectly good thin clients if we had virtualized desktop systems in our datacentre.
If the recipient wants Windows, the linux install at least shows that the hardware is OK, and how he re-installs Windows is not our responsibility. But with a bit of luck he plays with Linux a bit first, and discovers that he really doesn't want Windows at all.
Linux runs quite happily on old-ish PCs with 512Mb RAM (or even just 256Mb) that were struggling with XP SP3 and which are totally useless for Vista or 7.