If your machine powers on but Windows does not load properly, the first step is to try restarting in Safe mode – Start the machine, and hit F8 while it is starting, you should get to a text screen which gives you a choice of Safe Mode or Safe Mode with Networking. Safe mode only boots the machine with basic Windows software, and allows you to troubleshoot or recover files. http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/boot_failsafe.mspx?mfr=true
If there is an option for it, choose the “Last Known Good Configuration” http://support.microsoft.com/kb/307852/en-us
Or choose a System Restore point back to a date when you know the machine worked http://support.microsoft.com/kb/306084/ System Restore will not erase your data, but it will uninstall any software or system updates you made after that date.
If that doesn’t work:
Repair the Registry http://support.microsoft.com/kb/307545
If that doesn’t work, start the machine from the Windows XP disk and do a Windows Repair installation. http://support.microsoft.com/kb/315341/
Microsoft list of things to do if Windows doesn’t boot http://support.microsoft.com/kb/308041/en-us
If you can boot, check the SMART status of your hard drive to see if it may be a failing drive http://www.computerhope.com/jargon/s/smart.htm
http://www.ariolic.com/activesmart/hard-disk-smart-status.html
If it is a hardware issue, address the problem or take the machine to a technician
Once it is running:
Scan for viruses and trojan horses and clear caches in Safe mode
CCleaner (cleans out caches) http://www.ccleaner.com
MalwareBytes http://www.malwarebytes.org/
Superantispyware: http://www.superantispyware.com/superantispywarefreevspro.html
Avast! 4 Home: http://www.avast.com/eng/download-avast-home.html
Update the device drivers for your hardware by visiting the manufacturers sites for your motherboard/computer, video card, and other devices, and downloading and installing the latest drivers for your model.
Make sure that you are running up to date anti-virus software, and keep your Windows OS and other software like Adobe Flash and Web browsers completely up to date.
If a crash happens always while running a specific piece of software, uninstall it and reinstall the latest version of that software.
If a crash happens only after the machine has been on for some time, and only when the machine is under load (watching videos, playing games, doing compute-intensive tasks) then it may be a heat-related hardware problem. Make sure the machine is clean of dust, and that the fan(s) and air vents are functioning properly.