<apex:pageBlock id="theBlock">
<apex:inputCheckbox value="{!theSObject.CheckBox__c}">
<apex:actionSupport event="onchange" rerender="theBlock"/>
</apex:inputCheckbox>
<apex:inputText value="{!theSObject.TextField__c}" disabled="false" rendered="{!(theSObject.CheckBox__c == true)}"/>
<apex:inputText value="{!theSObject.TextField__c}" disabled="true" rendered="{!(theSObject.CheckBox__c != true)}"/>
<!-- Or to simply have a field that appears only when the box is checked -->
<apex:inputText value="{!theSObject.TextField__c}" rendered="{!(theSObject.CheckBox__c == true)}"/>
</apex:pageBlock>
hi check it once...
Change the code accordingly per your requirement.