Computer cycles all the time?

On starting my desktop computer the HD cycles before producing the black start up screen. The green DVD light cycles with it. Even after this phase the computer then tries to restart?
When it does start after a while the computer restarts of its own making.

What can be causing that and anybody recommend a troubleshooting approach to isolate it?

I thought I had a problem with XP and I am trying to reload it but with this restart problem I cannot complete the XP reload.

 

didnt get wat exactly u r trying to say.....can u please bit elaborate it??

 

What would you like more of?

I switch on computer and HD light cycles in sympathy with DVD green light. After a period the computer 'starts' but then it goes back to the start position.

I thought it was a problem with XP so I reformatted and was trying to reload XP from original disk/CD. The load up goes until the final element and then the computer restarts?

 

Memory could be the culprit in this case..... check your memory with [link=http://www.memtest.org]this program[/link] and see if you are getting any errors>>

 

Cheers I will give it a go
I may have to wait a while as away all w/e's before Xmas