It's Windows 8.1 that's the real problem (no compatible drivers available), but I had a lot of trouble getting fastboot working on my Windows 8 laptop too. You could try downgrading to Windows 7, which should be a lot easier for you, or persist with Windows 8, because in the long run you probably want fastboot working on it.

Sent from my Evita
HTC Evita | CandyKat | Torched KK
S-off | 2.18 | TWRP 2.7.1.0
Do not PM me asking for assistance, that's what my Q&A thread is for!