record button in Audacity activates accelerating loud sound

I have recorded from my keyboard using Audacity many times. Today something has gone wrong. The second I hit “record” a terrible beating, screeching sound starts and escalates rapidly. It does this even when I have removed my keyboard’s input. I don’t know where to find the version of Audacity, but I have used it for about 3 years now. My computer is a Dell XPS, Windows XP professional SP2. I use Realtek HD Audio manager. I connect my Casio CTK 611 keyboard using audio input. The problem seems to be with Audacity. I have burned cds to use in my kindergarten music programs by recording my keyboard into Audacity. Help!

You can find the version number in the Help menu > About Audacity.
3 years is a long time in software development so I’d highly recommend that you upgrade. The latest 1.3.12 version is available from here:

My guess is that the noise is caused by feedback through your sound card.
In Audacity 1.3.12 there is an option in the “Transport” menu called “software playthrough”. This should normally be switched off.

If that does not fix it then it is almost certainly a sound card setting.
In the Realtek HD audio manager, ensure that the recording input is set to the “Line” input and that “Stereo Mix” is not enabled.