im using a n95 8gb and i tried to install an application but the phone reads 'certificate error, pls contact the supplier'.

ive already checked the settings in e app. manager>>settings>>software installation>>All
online cert. check>>off
no default web address.

and how do i reinstall an app. like tiger woods 07. coz e game got deleted after i updated the firmware.

Just follow the instructions from this link:
http://symbianwebblog.wordpress.com/2007/11/27/8/

As for reinstalling, if the application was installed when you bought the phone and you upgraded the firmware, it's unfortunately lost for good.