Controlling forms based on a variable value

[Debstep]

I am "stuck" on using the forms a little. I have created custom forms in my dialog sequence and there are text fileds that display a variable, when the user changes the value, it writes it ( ideally ) back into the variable.
once these variables are set, I run a dll that verifies the values entered and I would like to disable the next button on the form if the validation fails.
Fir instalnce if my variable $SERVERNAME$ == (blank) then disable the next button or if my DLLRETURNVALUE not equal 0 then disable next button.

My question is this, where do I put the code that will enable\\disable the next button? Is it in the object rules section of teh form or in the code beneath the display dialog, in which case, how do I access the NEXT button control??
Any help gladly appreciated

thanks in advance