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.

Have you checked your DNS settings?

Everything is set on "auto", so the host machine should be passing both IP and DNS info on to the client machine (at least according to every "how-to" I read). I did try manually setting the DNS server IPs to those provided by my ISP, but it didn't make a difference.

Ad Hoc's are more than a bit dodgy, try setting everything as static addresses. Also what were you using as the gateway, the router IP or the ad hoc server IP?
Can you ping the router? Can you ping google (pinging a website by name is a good test of dns resolution too)?

All are members of the same workgroup??

The default gateway and ad hoc host IP has to be 192.168.0.1, ad hoc networks don't work at all if you set it to anything else (trust me, I've tried everything else). I can't ping anything outside of my home network; my router and other machines connected to it are no problem, anything on the internet, not so much.
No, the machines are not part of the same workgroup and that cannot be changed. The offline machine is actually my wife's work laptop and it cannot be "unjoined" from the domain it belongs to (well, it can, but it will be a pain in the ass for her to rejoin when she gets back into work). That shouldn't matter though.

Try assigning your other computers to the same workgroup name as your wife's computer. They will never actually be part of her workgroup but it may allow you to connect them all when they are on your network since they will see each other as the same workgroup. Just a thought . . . =)

Workgroups and domains aren't the same thing. I can't create a workgroup that will be seen as the same domain as the offline laptop.

So the guest laptop is getting an address from DHCP and can ping the router fine? You'll have layer 3 connections then, what sort of router do you have? Might be useful if I can have a look through screens of the config
Have you got the access to create a new profile on the wife's laptop? If not, downloading a liveCD of Ubuntu should let you set up a test system just to eliminate the OS as the issue. If you can use the net with Ubu then the problem is the configuration of her installed OS, in fact its more than likely NOT a problem but security set up by their IT department

That's what I was thinking (the security thing). The other most likely possibility is the router config, but I've barely customized it beyond the default settings. I would hope access like this is not something that was disabled by default. The live cd is a great idea (now why didn't I think of that), I'll give it a shot tomorrow evening.

OK, did you actually "share" the connection?
Open Network Connections on the host computer, click the connection you want to share and under network tasks click on change settings of this connection.
On the Advanced tab select Allow other network users to connect through this computer's Internet connection.
Then the client computer(s) should now receive a private class, non-routable IP address in the 192.168.0.* address range via DHCP from the host computer and should have full Internet connectivity.
It can also depend on the wireless network card as I've heard horror-stories about things not wanting to act as a AP because of firmware issues.

Its possible that the connections aren't bridged, there should be a bridge connections option somewhere in the network config, possibly selecting both and right clicking (not on an xp machine at the moment)
http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/hnw_bridge_install.mspx?mfr=true
try that

I've not much experience with ad-hoc network configurations. The device that is wirelessly connected to the host computer, will it show up as unique device with its own MAC address in your Router configuration?
Maybe a strict rule set in the firewall configurations of your router which prevents the laptop from getting an IP address attributed?
Most of the times I've had a connection stopped at router level was because of incorrect default gateway, submask, dns settings.
Also in some weird instances disabling a network device and reactivating it has solved soem problems.
But if that was the case (connections not being bridged) would he be able to reach the router at all?

Bridging the connections actually breaks connectivity for both laptops, so that can't be the solution (the connection loops back on itself).
The client laptop does show up in my router with a unique MAC address, but it appears to have the same machine name as the host laptop. It would seem that it has to be something about the IP address/DNS/gateway/mask, but there doesn't seem to be any obvious error in it. Like I said, by all rights, the setup I am using should "just work".