I want 2 rebuild wifi network from scratch

My network has been solid for a few years but now I'm finding odd things.
1. No instant connection, when I type password in it tells me it's the wrong password, but after turning airport off and no a few times it will accept the password.
2. My devices tell me they are connected but they still don't connect.

I tried re-setting up my network with new a new password, it worked great for about a week then drop off problems.

Also sometimes it will accept my old password. (this sounds like the problem)

I want to set everything up from scratch, whats the best way to go about this?

Devices are: 2 x Macbook Pro 10.6.3, Apple TV, PS3, Wii, Airport Extreme running two printers and hard drive, Airport express running speakers, Two iPhones.
All have worked faultlessly for close to two years.

Does your AEBS have the latest firmware? 7.4.2 and have you changed anything? Also
are you sure you are trying to connect to your router and not your neighbors. And last but not least if the channel is set to auto take it off and select 1 through 11 and keep in mind 1,6 and 11 do not overlap also very helpful download iStumbler and check the signal to noise, signal strength and how crowded the channel you selected is.

Usually when I had flakey network issues it was because of my Airport Express. That is - my computer would hook up to my airport express - but the express wasn't hooked to the main network so I was connected but couldn't really do anything.

Did you update firmware in either device? I found also - that the new firmware for the Express has a MAC address filter that you have to setup alongside the Base Station. So even though the Base station had MAC filtering on - You could still get in through the Express unless you put the same exact MAC filtering there.

Anyway - if you want to start from scratch - you could do a factory reset on both the express and extreme
Resetting an AirPort Base Station or Time Capsule FAQ
How to do a factory reset on the Airport Express-N - Mac OS X Hints
Then you'd have to reconfigure everything all over again.