What is the old computers mainboard specs?
Here's What You Need to Use Windows XP Professional
• PC with 300 megahertz or higher processor clock speed recommended; 233 MHz minimum required (single or dual processor system);* Intel Pentium/Celeron family, or AMD K6/Athlon/Duron family, or compatible processor recommended
• 128 megabytes (MB) of RAM or higher recommended (64 MB minimum supported; may limit performance and some features)
• 1.5 gigabytes (GB) of available hard disk space*
• Super VGA (800 x 600) or higher-resolution video adapter and monitor
Is the fan still spinning? try checking your power supply also, the fuse or some other component must have been blown.
Its very important to note the minimum requirements of Windows XP before an upgrade.