Problems Shutting Down And Restarting the Computer


I have just connected to Broadband using a Voyager 105 modem. Since I installed the new software for this, my computer won't shut down properly. The screen goes blank, but the computer doesn't turn off,(it carries on humming.....). Then, when I try to turn it on again, it won't turn on. Instead, it starts 'revving up' in an alarming manner, as if it's about to take off....The only remedy is to unplug it and start again. It doesn't do this every time. Last night it turned off and the modem turned off with it. This morning, it did its funny turn and the modem stayed on.
You will gather from my description that my technical knowledge is very basic. I would be very grateful for any help or ideas. Thanks.
The operating system is Panther.