Panda 300mbps wireless adapter problems

I have a Panda 300mbps wireless adapter. I used it on an old machine running windows vista and got really good speeds(20-25mbps). I just built a new machine running windows 8.1 and now the highest speed I get is 2mbps. I actually once got a speed of 0.09mbps. This is atrocious. I need to know if this thing is dead or if it is a software issue. I updated to the latest driver, even though with windows 8.1 it is basically plug and play. I did an internet connection test with my ps3 and I got 10mbps. It is literally the same distance from the router. I did a test with a mac pro from the same distance and I got 15mbps. The worst part is, I have a 5 bar connection the whole time.

I bought this adapter almost a month ago and it shouldn't be dead already.

It's not incompatible. Upon research, I found that windows 8.1 has issues with wireless adapters. With this adapter I didn't even have to install any drivers. Windows 8.1 automatically recognized it. That's why I said it was plug and play. The internet speed is really slow. I can't find a way to fix this. I can't plug it into the router because the router is in someone's bedroom.