computer switches off shortly after loading Windows

After having booted up my PC successfully first thing each morning, within the first few minutes it will switch itself off and often attempt to re-boot (always unsuccessfullly). I get into Windows OK and can launch an application OK, e.g. Internet Explorer, but it will still switch off within those first few minutes. I find that if I leave it in this 'switched off' state for about 10 minutes and then re-boot, it loads up fine and works OK for the rest of the day. Subsequent re-starts also work OK. However during those first 10 minutes or so in the morning, any manual re-boots hit the same problem, i.e. the PC swicthes off. A bit like a car having trouble starting on a cold engine. Once it is warm enough it starts and runs fine. I've checked the Event logs and no errors are shown. I've searched for .dmp files and none are found. Do you have any suggestions please? Thanks

Is this a new problem? Did it work OK previously?

What is the make and model of the computer?

The PC was built to my specifications by a local PC shop about 2 1/2 years ago. It has been running fine until now. It has an AMD Athlon 2800+ processor with 1/2 Gig ram and 120gb hard drive. It has a Radeon 9600 256mb graphics card and Realtec AC97 Audio. Runs Windows XP Home edition. When the PC is up and running it runs smoothly and performs well all day. N.B the same problem happened this morning - i.e. a click then a black screen about 1 minute after Windows had started OK - I waited about 10 minutes then manually re-booted and all I running fine. Again Event logs show no problems.

There isn't a lot I've added to the base build. Norton Internet Security 2007, Microsoft Office XP (Word, Excel, PP, Outlook), Canon printer s/w, Microsoft cordless mouse and keyboard s/w and Canon digital camera s/w. And that's about it. I can't understand why this 10 minute or so wait makes all the difference as I assumed that the same s/w is loaded each time in the same sequence (if that is the problem).

Go to start Run... msconfig | Startup

Everything listed here has been added by third party programs so can be disabled (and also re-enabled).

This is a troubleshooting tool. You can disable everything or go about it selectively to test for any changes in boot behavior.

I would start with Norton. Norton software is very unfriendly to Windows.

The problem hasn't occurred over the last 2 days. No idea why, but if it does re-occur I shall try disabling the third party start up routines as suggested. If I hit problems, I'll post another thread, but in the meanwhile thanks for your help.