Computer beeps when 3 keys are pressed

[Danny]

Ctrl+Alt+Delete. Very common keys to be used together, but whenever I try to press them, my computer starts beeping and Task Manager doesn't open. This usually happens when you press down 4 or 5 keys at the same time, but not 3.

Is there any way for me to fix this? Sensible replies please, no 're-install windows' because it was working a few days ago, only happened recently.