My computer restarts for no reason

Hello,

my computer cuts off when ever i hook up the modem to it, when i access the internet, it only works with certain. I can't get on myspace for nothing but 10 minutes or it will restart. I just got CA Security Suite 2007 along with my internet. I have windows vista and a 4400+ processer. PLEASE HELP FAST BECAUSE MY MOTHERS COMPUTER CLASS IS STARTING UP AND SHE WONT BE ABLE TO DO IT. PLEASE HELP!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!