My mac osx 10.3 panther is now shutting off when I am online--
This is new & I don't have a clue as to why.

Is it a desktop, or Powerbook. Dose it shutdown only when online?
Shutdowns are primarily caused y one of four reasons.
1.Loss of power
2.Overheating
3.kernel panic (though this usually gives you a screen of death)
4.A hardware error

1.Check power connections, make sure everything is properly plugged in by unplugging and plugging in again. Check that the power cable is not pulled tight but that it has some slack in it. Check that you are not plugged into a circuit that is over loaded. Avoid placing you computer on the same circuit that a air conditioner, laser printer or hair drier or any other item that uses large amounts of power are on. Use a serge protector.

2.When your computer shuts down feel the case to see if it feels hot. Fell the case during normal operation to have an idea of normal operating temperatures. To prevent overheating make sure that your CPU is in a area that allows airflow, check to see if vents and fans are free from obstructions and use canned air to blow dust from vents, fans and the interior of you CPU.

3.Run your Apple Disk Utilities on the hard drive Repair permissions and Verify the disk. Yo will not be able to repair the boot disk (your hard drive) because the utility needs access to files that it can not access when your computer is running off the disk as a boot disk. Verifying will let you know if you have any major problems. Utilities like Disk Warrior will be able to repair the disk by booting from the CD ROM. Other members may use other third party disk utilities and be able to tell you about them I use Disk Warrior because it is the standard where I work. You can also boot from a external Macintosh system drive and repair the Disk using Apple Disk Utilities or “T” boot the mac in question and hook it to another Mac running the same processor type and run Apple Disk Utilities on the “External” drive.
4.If all recourses above fail to fix the problem consult a Apple Certified computer repair technician and they will be ale to run a set of diagnostic applications that will find any hardware issues or assist you in finding any software issues you were not able to address.

May be due to virus

Quote:

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Originally Posted by gsbond

may be due to virus

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Sadly you may have a valid point, though a virus is a possibility, there is a much lower number of virus infections in Macs. Just a smaller market & better security makes it less cost effective to write viruses for Macs. That said running a AV program is still a must do. The lower rate of virus in the wild for Macs and the Unix based OS along with it's software installation methods make the thought of a problem being a virus a little further down on the list. Still if you have a computer Mac, Linux or Windows and all other OSs run a AV program and keep it up to date