Well, it would appear somebody has broken onto your wireless LAN (I am presuming that's what you describe by DLink G WAP). Presumably "Dave" is the culprit. Frankly, if you have your security correctly established, I can't see why "Dave" would bother to crack your WLAN unless he's incredibly bored or extremely cheap. WLAN with proper security installed, is difficult but not impossible to crack. Mostly time-consuming. I hardly see setting up "Davesgamehouse" as being worth the effort.
However, it could be just the opposite, your system is picking up Dave's who hasn't bothered to enable any security. Hard to tell from what you've related.
And, of course, there's always the potential its an inside job. Setting up a workgroup from inside is simple enough.
First of all, you need to figure out what security you actually have enabled. I don't like to cast aspersions, especially at someone I know only as your "Network Guy" but its possible you don't have any security set up or its set up wrong. If I am wrong on this, a thousand pardons to your NG, I mean no offense.
If indeed your WLAN has the full security measures enabled, there's not a lot more you can do. But like I said, Wireless is basically not that secure, it can be compromised by virtually anyone with substantial skill, means, time and determination. But generally, its not worth it just to fool around, unless you are dealing with a sociopath.
One thing is the culprit is most likely within a 100 yards of your system, probably less. Or it was a drive-by attack. This is called "war-driving". Have you noticed a car or van constantly parked nearby that didn't used to be? If so, maybe take a look in the windows. Or has someone moved in near your offices lately? What's around your offices? You may be able to find "Dave" and let him know you are on to him and could have some voluteers help him voluntarily see the errors of his ways. Or perhaps not so voluntarily.
If you find your system's security is not fully enabled, you will want to do so immediately. First of all, change the admin and other screen names and passwords. The admin name and passwords should then be known only to yourself and your Network Guy (have you considered the possibility this may be "Dave"? Again, no offense intended, just being real). Then enable WEP with 128-bit. If your system has it, enable WPA. And lastly, enable MAC address filtering. If you have any other security, enable it all. All of these are relatively simple to do, though they can be frustrating at times. With all enabled, "Dave" should be able to find much better things to do with his spare time. If you identify "Dave" as a neighbor and are unable to convince him to leave your system alone, perhaps you can re-configure your physical setup to cut down on his access, place your transmitters so there's good solid walls between them and "Dave", maybe even set up a foil screen in his direction to block him out. Or you could take matters into your own hands and give "Dave" a bit of his own medicine. Two can play.
In any case, it would behoove you to educate yourself on the issues. Check out:
The Ars Technica article on Wireless Security, http://arstechnica.com/paedia/w/wire...802.11b-1.html
Here's another good site with lots of information though its a bit more technical:http://www.drizzle.com/~aboba/IEEE/
and here's some more: http://www.wlana.org/learn/security.htm
As you can see from some of the dates on some of these, WLAN is not a new issue.
There are plenty more. Just google on WLAN Security, WEP, WPA, EAP, etc.
But in most cases, changing your SSID, shutting off broadcasting, changing your admin and user names and passwords (and keeping those secure, especially those with admin authority), enabling WEP, WPA, and any other security measures, plus using MAC address filtering should keep you fairly secure. Monitoring your surrounds can also help.
Lastly, you need to do a full spring cleaning of your entire system. Who knows what "Dave" may have installed on your machines? If you can do a system restore with XP, do so to before the problem appeared. Take a ghost or secure whatever data you have accumulated since it started onto CD or DVD so you can sanitize it later as well as have evidence to give the authorities should you find "Dave" and want to press charges. Then do a full sweep with Adaware, SpyHunter, a Registry Cleaner and anything else you can figure out. Hopefully that will find anything "Dave" may have left behind. While doing this, make sure the LAN is down and the machines are unconnected so you aren't just chasing the problem around and around. Then set sharing rights on all machines and drives and sensitive folders and install passwords. Again, keep those secure.
If you are lucky, "Dave" will have been just a passing problem. If not, like I noted above, he's got to be within a limited radius and you could conceivably find him. If so, remember to report him/her/it to the authorities and definitely press charges.
Lastly, its possible "Dave" didn't come in over the air waves. Its possible "Dave" broke in via the internet connection. I can't see why he would then set up a domain on your wireless, but maybe he's really bored and was just trying to see what he could do. You may want to review your existing firewalls and perhaps upgrade them. But I would bet "Dave" is an air pirate, not coming in off the internet.
Good luck and let us know what becomes of all this.