XP Pro shuts down

My desktop Acer, running XP Pro SP2 boots, gets to my desktop then shuts down and reboots. The cycle continues. I’ve tried restoring, but to no avail. This started this morning and the only difference between yesterday and today is that I downloaded and installed Yahoo messenger. I have since deleted it, but that didn’t work. I am able to boot in safe mode, no other way. Suggestions? Thanks in advance.