From the symptoms you described, it is likely a software issue. Either virus, missing system file, missing driver or corrupt registry.

There are several things you can do:

- During post press either F2, F12 or Del key to enter BIOS setup. In advanced menu set "quick POST" on "disabled". That will make your system to do more thrall POST, possibly report an error. If any messages occurs, please post them.

- Press F8 while booting and choose safe mode. There is a tool called event viewer (part of the administrative tools) that can help point out the cause. It keeps record of any error messages. If you find anything helpful, please post it.

- Any recent hardware change or driver upgrade can cause this behavior. Windows XP are next to impossible to repair if the damage is more than minor. Especially the registry. Safest way to resurrect your system is to reinstall the windows. To preserve your work be sure that you don't choose formating the drive before installation begins nor do anything with the partition where your work is. You will need to reinstall the software needed for your work too.

- it is possible that a hardware failure is causing this. If it is so, memory is the suspect no. 1. Suspect no. 2 is motherboard. Also, you will most likely not be able to reinstall the windows.