|"Its maximum output power is 300w"|
That's pretty low by modern standards. You didn't bother to post the make/model of ANY of your hardware other than the motherboard. We have no idea which CPU, which graphics card, how much RAM, how many HDDs, etc. There is no such thing as a "core i2" but based on your board which is a S775, you probably have a Core 2 Duo or Core 2 Quad.
The beep code is the key & you stated there's repeating short beeps - that indicates a power problem. Wattage requirements vary depending on hardware configuration but the PSU will only supply what is needed, so if your system only requires 250W, having a 1000W isn't a problem as it will only produce 250W. Having a 300W unit might be a problem because it could be running at or over max capacity, depending on the unit's efficiency.
Since you didn't post the PSU info here's what you should generally look for in a decent quality unit:
- 400W minimum
- single +12v rail at 30A or more. Units with multiple +12v rails should be avoided.
- +3.3v & +5v rails at approx 20A each. If the +5v is significantly higher, you have an outdated PSU based on an older ATX standard
- active PFC (power factor correction)
- 80 plus certified (efficiency)
Here is an example of a well spec'd Corsair 430W unit: http://www.newegg.com/Product/Produ...
And here's a poorly spec'd X-tec 400W unit: https://3dprinting4u.files.wordpres...
EDIT: another possible cause of restarting could be the heatsink is installed incorrectly on the CPU. Most CPUs are very fussy about thermal paste application. Too much, too little, or improper placement (ex. horizontal line used when vertical line is required) can prevent a system from starting. Make sure you applied the paste correctly for your CPU.
message edited by riider