Why can't I access my NAS box (with a photo album server) from the internet
when my pc is in standby mode?

I have a DSL connection coming into the DSL box, which is connected to a
NETGEAR router. Connected to the router are a desktop pc (Win XP Pro SP2)
and a QNAP ts-209 Pro NAS box which also has some server software (photo
album, ftp server, media server). I'm currently only using the photo album
feature. I've set up a web address to access the NAS. I use a dynamic DNS
service to account for my ISP chaning the ip address. I've set up port
forwarding on the router to point to the NAS and proper port. I can get to
the photos fine from the internet when my desktop pc is up and running, but
not when the pc is off or in standby mode.