Windows XP: computer starts up, then shuts down and restarts.?

This is usually on the first couple of screens after switching on - gets caught in a vicious cycle loop of switching on, going so far, powering down, switching on again, getting so far, powering down... what is causing this and how to put it right?

When issue started, I was reporting a dubious poster to a systems admin on a site I frequent - I cut and pasted the offending email intending it as evidence. The offending email had a link in it, but I was not clicking on the link directly - is even copying and pasting an email containbing a dodgy link enough to activate a virus?