Power Mac G5 on start up it repetedly has white screen with circling icon then just switches off

If i press alt when switching on i get a grey screen with 3 icons, 1st is like a refresh arrow button & last is a straight arrow button on pressing either of these mac just starts the white screen thing again, the middle icon is a picture of a hard drive with a blue cross in the corner & written under neath it says macintosh HD.

That is the Startup Manager screen.

Click the Macintosh HD icon then click the right arrow.

Does the machine boot after that?

It sounds like the hard drive could have failed.

Can you boot to an OS install disc in the optical drive?

Hold a mouse click when starting to open the optical drive.

You can attempt hard drive repair using fsck in single user mode:

http://support.apple.com/kb/TS1417

Been there, done that. Try a Safe Boot (holding down the shift key at boot) and see if it boots. If it boots, then you may be having problems with either the power supply or the video card. What video card is in the machine?

If it does a safe boot OK, then we can troubleshoot using a verbose boot. Verbose boot puts up a Unix Console screen and displays text output during the boot process. When I had these symptoms, verbose boot showed me that the power supply was clicking off right after the kext to drive the GeForce 6800 card loaded. I replaced the power supply and the problem went away.

To initiate a verbose boot, hold down the cmd-V combination until you see the console screen come up. Watch the output and pay attention, because you need to remember what is being said when the machine shuts down. You may need to repeat this two or three times.

As an aside, often times power supplies exhibit this behavior when they have become choked with dust. This is pretty universal after five or six years, requiring that the power supply be removed and the dust blown out with clean shop air.