1 make sure the service is listening on port 5900
at the original machine type
telnet 127.0.0.1 5900
2 make sure the port on the router is forwarded to the correct internal ip
from the remote ip type
telnet xxx.xxx.xxx.xxx 5900
did you restarted the router after you made the changes?
any firewall on the debian box?
any firewall on the windows box?