Hi giacomo - Welcome to w7forums
I just had a look for that modem and did not see any drivers for it on the manufacturers page -
http://www.speedtouch.net.nz/st530.htm
There is a firmware update version for download. Have you tried them to see if they help?
Another question that would be helpful.
How are you connecting to this modem?
Are you connecting through USB or Ethernet port?