I have win xp sp2, running ie6, ff1.5, and opera 8.5 browsers, and quickheal antivirus (latest version). i dont have any 3rd party firewall. lately , i have noticed that the firewall does not start when i boot up, and i am unable to manually activate it, as the control remains grayed out (the check box in the control panel).

how do i activate the firewall?

This is generally caused by certain services being disabled & or improper permissions.

Start > run > services.msc and go down the list looking for Windows Firewall ICS and the and make sure that service is not disabled.

Also:

You cannot start the Windows Firewall service in Windows XP Service Pack 2