This might be an antivirus issue, so if you have an av then you could try disabling it temporarily, or indefinitely. You can also try starting ftb with administrator privileges by right click and choosing start as administrator; this option might not be available so you may have to run the command prompt as an admin. Open the start menu and type in cmd, then right click it and choose run as administrator, then start ftb from there by typing "C:\Program Files\Java\jre7\bin\java.exe -jar " without the quotes and with a space after -jar, then dragging in the ftb jar or exe and pressing enter.
For all of the times when I say to choose start as admin it should show a prompt with the title "User Access Control", unless you have disabled this in the control panel, which you probably haven't.