600 watts regulated DC for 3 seconds
The BEST way to solve this problem is to get an AC mains UPS (uninterruptible power supply) that is able to supply AC power to your computer during momentary power outages.

You can get UPS units that can supply 650VA which should work nicely for your application that are less than $100. The beauty of these units is that you simply plug your computer into the UPS and then the UPS plugs into the AC mains. 

I would strongly encourage you to stop thinking you can kludge together some type of home hack solution on the DC side of the supply. You will certainly spend at least as much in parts and the solution will be a hack at best -- if you know what you are doing when you built it.