[SalesForce] how to make field ready only..

This isn't possible on standard pages using validation rules - the red "required" marker is only displayed when a field is required, either at the page layout or field level. A validation rule will mark the field when the error occurs, but this is after the user attempts to save.

You can do this in Visualforce, by conditionally rendering some additional classes, but that would mean (a) moving your whole page to Visualforce and (b) having some of your logic in the validation rule (the actual rule) and some in the page (the knowledge about whether to display the field as required to a particular user).

If your rule was specific to a profile rather than a user, you could have a different page layout for each profile and make the field required (or not) at the page level.

You're going to have to either adjust your validation rules or re-look at your rollup summary fields. Regardless of the criteria in the roll-up summary field, an edit to a child record forces its parent to rerun triggers, workflow, validation rules. Take a look at the following document on tips for preventing related validation rules when the parent record is impacted with rollup summaries: http://help.salesforce.com/HTViewHelpDoc?id=fields_about_roll_up_summary_fields.htm&language=en_US