Computer (Dell Vostro 200) freezes, long beep, must restart

For the past several weeks my computer freezes up while I am working.  Usually, I have 2-3 windows open (but not always) and most often I am working on the Internet (but not always).   If I step away from the computer and have windows minimized, when I come back the clock has stopped and when I click on the minimized window to open it it turns dark blue, the computer freezes, there is a long beep and I must restart the computer.  I can't get anything done and am ready to toss the computer out the window.  It took me two restarts to open this thread and post ... Any thoughts on what could be causing this? 

Thanks for your help!
Beth