microsoft software not genuine?

[Technologist]

Can anyone explain how it might be possible that a valid piece of software became invalid without the PC leaving the house, and if buying a new version is my only option?

Two possibilities -

1: Someone else is using your license key.

2: You've modified your machine to a point where the OS thinks it's been moved to a different machine.

Try reinstalling your OS from the original media and then re-apply the license key from the certificate you go with it. This may be on the disc sleeve or on a sticker under your machine.

If you still have the problem, contact Microsoft & sooner the better as they're going to shut down XP support soon.