Wi-Fi Tips:
Just remember to not to cover up the right hand side of the Desire as that is where the Wi-Fi aerial is housed. Makes quite a difference.
Have you also checked that there is no other Wi-Fi signal in the area using a similar channel? I use the Wifi Analyser app to do this.
Have you tried turning off the 'b' part of the Wi-Fi via your router?
This can free up traffic.
If your router has two aerials, make one of them horizontal, the other vertical. This helps bouncing the radio waves through rooms.
Is there a wireless video/audio sender in the area? These often run at 2.4GHz and can seriously affect Wi-Fi.
You can can either add silver foil dishes to the wi-fi aerials to concentrate the waves towards an area of the house or if your aerials are detachable. Add better aerials.
Lastly if all fails, get yourself a Wi-Fi repeater box. This relays and boosts the Wi-Fi in weak parts of the house.