You most likely forgot to make the USB boot on startup. Either that or you forgot to make the actual USB bootable and it is doing nothing.
Go into BIOS (if not Windows 8) and make the USB boot on startup.
If Windows 8; make the USB bootable, eventually search on Google how-to.
If Windows 8, here's a tip: You should seriously consider upgrading to Windows 7. Yes, I just said upgrade, because Windows 8 is the worst operating system ever and should not have been made in the first place, if anything Windows 8 is only a downgrade.