D seems to be the problem child. I would have to assume you do have internet assess on D correct? It would be nice if you had a third computer to ping D from to help pinpoint the problem but if you access the software on the router, usually under some kind of diagnostics heading, you should see a ping test. See if you can ping D from the router.
If D has internet access, D can access L but you can't ping D from either the router or L then you have some type of firewall blocking access to D.
Look closely on D to make sure you don't have any other third party firewall like ZoneAlarm or anything else. I just hit this same senario here a couple weeks ago with Norton. All indications were that the Norton firewall was off but the final resolution was uninstalling Norton. I hate to suggest it but try uninstalling Kapersky on D, make sure the Windows Firewall is off and once again look closely for yet another third party package that might be firewalling you on D.
