alarm sound on computer need help?? plz annoying

alrite iam sry if i posted in the wrong place but it is my first time
i been having problem with computer i bought it off my mate for $200
with 120 gb harddrive 700ram 128 graphic card it is all good
the think is that sometime it playing a warning alarm and it woudnt stop till i restart computer you known when the alarm goes off at a house just like that
but lately my computer as been freezeing and plays the same sound and i have to shut it down
i know it a hardware problem and i am a student cant ask anyone to have a look at it
but i am pretty sure i can do it myself just wanna kn if anyone out ter has a good idea of y it is making tis sound ? my graphic card is raddon 9000 or wateva also have a normal sound card win 2000 tv tuner 1 asus uietrack cd player and a lg did buner that is all i kn alrite now so anyone knowns anythink it wud be a big help thanks in advance