Hello,
I recently bought a HTC Desire and also had a WIFI problem. I could not find any specific topics on HTC Desire and WIFI problems. In the end I stumbled upon this thread.
Previously I learnt how to solve a similar for the Wii. This thread taught me how to access advanced WIFI settings on my HTC Desire. By changing the DNS settings on my HTC device I could solve the problem!
The solution is very simple: change the DNS on your HTC device to the IP address of your wireless router. This does not make much sense, but it works very well in my case.
This should work when you already can connect your HTC to the wireless router (i.e. your HTC device successfully receives an ip address from the router), but you do not get a response when you enter an URL in the standard browser. A lot of people posting to this thread seem to have this problem, though there also some people who cannot connect their device to the wireless router at all.
Could some people try this out if it works in their case?