Power LED blinks red once every
1. Power supply overloaded or short detected. Unplug computer from
AC power source. Unplug internal power cables from all devices and
unplug PCI cards. Reconnect devices one at a time to identify the
2. Diskette drive power cable not properly connected. Ensure that all 4
pins on the drive are connected to the power cable connector.
3. Power supply shuts down after system warms up. Thermal overload
caused by bad fan. Replace power supply.
4. Power supply will not turn on because of internal power supply fault.
Replace power supply."
I got that out of a Compaq Deskpro troubleshooting guide.
Your problem is power related. I would try all the steps listed above.
If this system is still under warranty by the EBay seller, I would notify them about it. If you don't wish to do so but want it fixed, I would take it to a professional technician or tech savvy friend since you say you're not good with computers.