Okay, here's my situation: I have two laptops, both with wireless capability and both running Windows XP SP3. One of the laptops is connected to my TV and has a wired Ethernet connection, the other has no connection at all. I set up the wired laptop to share its internet connection and created an ad hoc wireless connection for the offline laptop to connect to. I can get the offline laptop to connect to the ad hoc network and can even go so far as to access the configuration on my router that the online laptop is wired to, but I have absolutely no access to the internet. I have tried with and without Windows firewall enabled and both a secure ad hoc and unsecured one but it doesn't seem to make a difference, the offline laptop can't get past the router and out to the internet. I've tried numerous how-tos on the subject and followed all of them to the letter (most are just copies of an old Microsoft document on the subject) and by all rights, this set up should "just work", but it doesn't. Anyone have any ideas on this? If you need more details, just let me know.

Oh, did you disable DHCP on your router? Since you assigned a static IP to your host and the wireless laptop?

DHCP doesn't need to be disabled to set a static IP on a client machine, as long as the mask, gateway and DNS are also set correctly. The machine that has a static IP just won't ask for an IP via DHCP. Besides, the static IP is set on the wireless connection, not the wired connection. The wireless connection doesn't directly interact with the router at all.

I misunderstood your earlier posts then, sorry. And you wouldn't want to try to set a static ip for both and disable DHCP just to be sure? (after that you would of course check if your wired computer can access the net)
Could you post the full network connection details of both computers? Maybe there's something we have missed here.

It's not really a complicated setup: a Linksys RTP300 router (provided by my IP phone service, so I can't replace it), set up as a DHCP router with NAT; four hardwired machines all connected via DHCP (desktop, file server, laptop and game console). Very basic and simple.
As for setting up static IPs for everything, that didn't make any difference.