What level of security are you using?
I have a DG834N and one machine on my network would not connect whatsoever whilst I ran security at WPA2-PSK [AES]. As soon as I dropped the router a level to WPA-PSK [TKIP] it connected no problem. Had me foxed for ages, I can only assume the drivers for the wireless card in the errant PC (one of the newest on the network ironically) just were not compatable with WPA2.
Just a thought.
Edit - re-read your post, if you are using WEP, then the above can be ignored. I'll leave the post in case it helps anyone else.