You may want to stop these services which you may not need, unless you are running any server:
>sendmail ----> if you are not running any mail server
>sshd ----> if you don't login to your PC remotely
>httpd or apache ---->if you are not running any web server
>mysqld ----> mysql database daemon
>samba -----> if you don't have any windows m/c in your network
>bind -----> if you are not running any nameserver
but you can start these services manually as and when required.
I had this problem of this constant hard disk activity, i use FC6, then found out it was the automatic system update program (yum-updatesd), it takes lot of time to search programs in my system that needs to be updated.... this may be your problem and you can disable it and manually update when required.