Pues según veo existe este detalle con el Windows 2003 y su IIS:
Cita: Windows 2003 (IIS 6) contains a property that limits the amount of data that can be received using a POST operation, and this has a relatively low default value. This property can be changed by editing the metabase.xml file and it is called AspMaxRequestEntityAllowed. If a file is uploaded that is larger than specified by this property a DNS error will result.
y
en esta liga hay unas soluciones para problemas similares, chécala a ver si alguna te sirve.
Saludos