This is because Waoo (assuming this is your provider, as its the only fiberoptic ISP in Denmark with IPTV) has defined VLANS in the packetfront box, and NOT routing.
You have set it up corretly. The above suggested solution would work, however the used bandwith when watching TV would be taken from your Internet bandwith, as they would be put on the same VLAN.
Your TV/Internet goes down, because your dlink router can't handle multicast traffic to wireless and "crashes".
To avoid this, log into your dlink and disable multicast traffic to wlan.
While you are it, on your htpc, change the metric for tcp/ip4 from automatic to something you decide, just make sure the LAN has the lowest value, eg LAN = 3 and IPTV = 5. These settings can be changed under the two network cards advanced settings for tcp/ip4. Reboot may be required.
Under dvblink settings, make sure your sources uses the network adapter connected to iptv (usually gets an IP in the 10.x.x.x range). Remember to clone your tv box MAC adress first.
This makes sure windows always prefers traffic via LAN, and TV via its own VLAN in your multihomed setup. This way you make sure you have your full Internet bandwith, no matter how many channels you are recording