How to combine multiple networks into one?

Here is what I have setup at my home. It works great for internet
sharing, but I want to be able to move files across every computer
rather than just among computers on the same router.

Here is my setup

DSL Model (Asante Equipment)
Asante FR3004LC Router
Asante FS5008 Switch
D-Link DI-614+ (wireless) (Upstairs)
D-Link DI-524 (wireless) (Basement)

We have our house wired so that the DSL comes in and is immediatly at
the Modem, which is connected directly to the Asante Router which is
connected to the Switch.

Then the other 2 routers are hardwired to the Asante Router (I think
they go to the router, not the switch).

What I want is to be able to set up everything to be used as 1 single
network. I want to be able to move seamlessly from one wireless
network to the next without being on a new network. Currently I have 2
wireless networks (upstairs and downstairs) and computers on upstairs
cannot see those on downstairs, and those on upstairs network cannot
see downstairs, and those wired into the Asante router are hit or miss.

I also have a real nice HP OfficeJet 7410 All-In-One with wireless
capabilities and can only use it on computers connected to the same
wireless router as the printer is.

This is real real real annoying and I want to configure them all to
work together! Please help me!