windows xp sp2 problem

Clarifications

It could be any one of hundreds of things. Without more detail, it's pretty tough to guess what is wrong.

One think you can do though is to boot into Safe Mode and turn off the 'Automatically Restart' on System Failure option. That way, you should receive a Blue Screen with lots of information on it. If you tell us, in detail, what the Blue Screen says, we might be able to help.

by pressing the Function Key F8 repeatedly on startup. Follow the steps below with the System started in Safe Mode with Networking.

Click Start, Run type msconfig and press Enter.

Now if you have the Configuration Utility open.
Configure selective startup options
In the System Configuration Utility dialog box, click the General tab, and then click Selective Startup.
Click to clear the Process SYSTEM.INI File check box.
Click to clear the Process WIN.INI File check box.
Click to clear the Load Startup Items check box. Verify that Load System Services and Use Original BOOT.INI are checked.
Click the Services tab.
Click to select the Hide All Microsoft Services check box.
Click Disable All, and then click OK.
When you are prompted, save the settings and restart the PC.

If it is working in Normal mode re-enable one item at a time until you find out what is causing the problems.
When you have it sorted out re-run the Configuration Utility and in the System Configuration Utility dialog box, click the General tab, and then click Normal Startup.

You could also check for Malware. Is your AntiVirus software up to date.