Bit Defender is rated quite highly in the tech magazines and on-line. Is this Bit Defender Plus? When she says her computer is faster, it's faster compared to what? What I mean is what anti-virus was she using before? If it was one of the resource hogs, that's probably true. I think you are using the free Windows Defender on your Windows 8 and that definitely won't slow down your computer. I can't remember what you have on the Vista, but if it's Microsoft Security Essentials; that won't slow it down either. So you probably won't get any speed-up boost from it.
But it's good anti-virus if you are not satisfied with what you are already using.
Fireplace, yarn, knitting needles, books, wine... check. Yup, I'm ready for winter.