Why does modem keep kicking off?

Turn off and unplug the case. Hol the power button in for about 30 seconds.

Take off the side of the case and use a can of compressed air to blow out all the dust. Do not allow the can to be tilted too much.

Use something to prevent the fans from spinning while you are blowing out the dust.

Then plug it back in and power it on.

Verify that the fans (power, cpu, case, video, etc) are operating properly.

If you feel you have the skills, obtain some thermal putty, remove the heatsink and clean it and the cpu from any old paste. Next apply a very light coating to the cpu, reattach heatsing and plug in it's fan connector.

It could also indicate a failing power supply. Test that and or replace with a known good one.

Hello,

Is it the modem that stops working? ie: the little blinky lights go out and the modem restarts itself? Is it your computer that sometimes lets you log into windows and then reboots? If the computer is constantly booting then you may have a virus. Heat may also have a part in the computer's performance. If it is your modem that is recycling; consider contacting ISP support to see if they can trigger your modem or at least observe the errors or erratic behavior. A quick look at the HP support site shows your computer is fairly old so you may want to consider reconfiguration. Clarified information and more information is needed before anyone can figure anything about your computer.