First you need to download hellocarbide, installserver 9.1 for N80,N73 and installserver 9.2 for N95, LonelyCat Xplore. You can get this file at mosh.nokia.com for computer or mosh.nokia.mobi for mobile web.
First step: install & enter hellocarbide and choose menu1. *make sure xplore is active in background because once you enter menu1, all application will not work until you restart your phone
second step: enter LCG xplore and extract installserver file to C://sys/bin
third step: restart your phone ane enjoy...no more certificate error when installing a application