I agree with everything g1g posted.
A regular household outlet in your home is 15 amps AC or 1800 watts. That gives you an idea of how oversized a 5000 watt inverter is.
A pure sine wave inverter would be better for your situation than a modified sine wave inverter. It will be better for both your computer and your microwave oven. Example: In a microwave oven, popcorn might take 4 minutes to pop using a modified sine wave inverter but only 3 minutes to pop using a pure sine wave inverter.
I'd suggest going to 1250 or 1500 watt inverter if you are going to try powering a small microwave oven. The 700 cooking watts probably actually uses a 1000 watts or more.
You might find this post: http://www.classbforum.com/phpBB2/vi...9&t=779&p=2371