Laptop with HDMI

Hello All

 I am a new boy here and hope you can help me with the following:

I have a Dell inspiron 1525 with HDMI output, it has the Vista OS. I output itunes via HDMI to my Onkyo TX-SR605 and it sounds jolly good too. However when initially setting it up I could not get a dickybird out of it till I realised that I had to set both the itunes and windows volume controls to above zero.

Hence I have 3 volume controls in the loop, this surprises me as I thought that "raw data" is sent via HDMI to be processed by the DACs in the receiver bypassing any controls on the laptop? Is there a way of deactivating  the itunes and windows volume controls when it detects HDMI is being used? Does it matter?

Thanks

Derek