802.11a is dog slow, but uses the 5 GHz band. 802.11b and g are faster, but use 2.4 GHz which is full of interference from microwaves, cordless phone, etc.
This is a VERY common misconception: 802.11a is actually the same speed as 802.11g. In fact, in many ways, 802.11g IS 802.11a, just specified to run at the 2.4GHz band instead of the 5GHz band.
Just to second this bit, in that yes, A is G, but A doesn't have channel bleed issues - there are only 3 channels that don't interfere with each other in 2.4ghz, 1, 6 & 11.
To Mr Surapon, check and see if your microwave is older, you may want to replace it with a newer, more shielded model.
For Mt Spokane, check out http://ubnt.com
- Ubiquiti Networks - their AirMax Bullet and Pico products are great for setting up links between buildings, or just burning through walls.