Does one need an antivirus with Windows 8?

Ok, so I got my mom to change her old-ass laptop for a Vaio Pro. When setting it up, I uninstalled some crapware, including Karspersky.

I did this because I was told that Windows Defender would be enough, as I`ve done in my own PC without any issues.

The thing is that the ultrabook started to show some keyboard and performance issues, so she (alone, I couldnt go with her) took it back to the store, and the tech support guy (not the sales guy) told her that this was all because I had uninstalled the preloaded antivirus. She believed him, obviusly, and handed over the device so they could proceed to repair it.

So the question is, was it my fault?

PS: Also, does anyone know if there is a way to get back one of the five Office 365 home premium license , so i can use it another computer?