Try with another USB 2.0 certified cable. Try plugging on other USB sockets. You could also try looking for the latest chipset drivers for your system (this website might help).
Also, when you plug the burner, notice if Windows doesn't say something like "this device can operate at faster speeds": if it says this then the device is operating at USB 1.x speed, which is why it doesn't go above 0.7x burn speed.
You should also enable Automatic Updates to update to the latest service pack for your Windows edition.