Side Note - s-on users
If you are s-on and don't want to s-off (for a fear of ruining your device), you can flash the boot.img by using this toolkit or doing the following:
1. Extract the boot.img from your rom (open the zip, no need to extract, and copy boot.img onto your computer)
2. Flash your rom as you normally would. When it is done, boot into BOOTLOADER (going to system will result in bootloop) and go into fastboot.
3. Type "fastboot flash boot boot.img" in command prompt using the sdk drivers. (If you don't know what this is, just use the toolkit)
4. You should see a green bar on the top right corner. If it is done, then you are reading to reboot your phone.
***Note that to do this, you must have adb debugging enabled. The cmd code also assumes that the name of the boot img is boot.img. If it is note, change the name or change the code"
S-Off Firstly, what is S-Off? This is a feature present in HTC phones that turns the "security off", menaing that you no longer need to flash the boot.img separately, and that you can change your firmware, radio, etc easily. However, note that the lack of security also means that you can seriously brick your phone if you flash the wrong rom. No security = more functions & more responsibility
There are three ways of achieving s-off. Before downloading, please read all instructions. Whatever you do to your phone is your responsibility alone
xda thread here.