I would not trust an out-of-box computer, possibly with a bunch of third-party applications installed, but an old computer which has been regularly used (including online activities) is not a good option too. Linux Live CDs have been mentioned already, and one of the best such systems created with security in mind is Tails (
https://tails.boum.org/). Make a USB stick with it, boot your regular computer from it with network cable unplugged (or Wi-Fi switched off), and you can safely generate a paper or cold wallet. It is certainly easier (and safer) that a clean install.