How can I do ? Is is the same if I turn off wifi on a more recent PC ?
Best option is to format hard drive and install free open source Linux OS, something like Fedora, Debian or Mint are OK.
You can even make dual installation with winOS if you never used Linux before and than install Electrum wallet on Linux.
I would not suggest upgrading to win10 or 11, they are privacy nightmare and they are much more affected by malware and viruses.
Any tips for generating seed offline without buying a new machine ?
You don't need to buy new machine for this.
One way of generating seed phrase offline is with using dices and computer that is not connected to internet to calculate last word.
Even old phone in airplane mode can be used to generate seed words with software wallet apps like Airgapped, but make sure to reset and disable everything.