Whenever i want to install a software downloaded from the net, it shows certificate error? what can i do??

You need signed applications(nokia 5800 compatible Applications
1) Make your phone memory backup data using nokia pc suit 7.1(very important)
2) After that remove your Micro sd card from mobile..
3) Remove your SIM card also(very important)
4) press *#7370# and unlock code to format your phone memory(yap.. this is safe default is 12345)
5) You will lose all your important date with this action...so make a backup of your phone memory
6) Now nokia 5800 xm will reboot.....wait for a minute....
7) Then select your country region as "Hong Kong"
Cool Set the current time......(set your phone to offline mode)_
9) Now you can put that Micro sd card ......
10) And don;t insert that SIm card right now...just wait....and try to install any supported singed application (E.G-nokia handwriting calculator)
11) And you may install any singed application ......
12) After that just install your Sim Card.