Device freezes after trying to reboot with bootloader

I have been trying to unlock my bootloader as per the instructions found here. I already have installed the drivers from Google USB driver folder that comes in Android SDK "extras" folder. However, when I try to start my device into the fastboot mode with cmd, it freezes. I type "**adb reboot bootloader**" to the terminal, as instructed. It seems to be trying to restart my phone, yet it freezes before it can actually start up, stuck on the initial HTC screen. Has anyone ever encountered an issue like this, or may anyone have an idea for a solution to this? I would manually enter bootloader menu and get my phone to fastboot if I could, but holding down the volume down and power buttons (as instructed ) won't help me at all. Pressing volume down and power simultaneously will simply start my device with a regular boot (and I have no means to "deselect Fast boot" - I simply don't have the option in battery settings). I am on Windows 7 laptop and my phone is HTC Desire 516 using Android 4.3.