So, I've had the same with my previous Sony Ericsson Vivaz which was Symbian S60v5 (or w/e the name is), way to get around this is to grab a fake or even a real anti-virus (norton for mobile or w/e) and get an app called 'RomPatcher'.
You must install the anti-virus, set RomPatcher files off quarantine (so they would be allowed to run), then you just Install RomPatcher, enable both options on the menu, and uninstall the anti-virus.
The above was the solution for Sony Ericsson Vivaz, not guaranted that will work the same on your Nokia RomPatcher will allow you to get around that certificate problem

After going around google searching RomPatcher and Nokia N95, i came across these articles; read them
http://www.martin-gardner.co.uk/how-to-hack-the-n95-to-allow-unsigned-applications/http://thenokiablog.com/2008/07/08/how-to-hack-s60-nokia-phones/http://www.finestfones.com/2008/05/rompatcher-easiest-way-to-free-your.htmlhttp://forum.gsmhosting.com/vbb/archive/t-558619.htmlFirst link, could work, but read all rest to be sure

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Anyways that certificate problem exhists, because in past you could get your self 2 files (.key + .cer) which would be generated from special sites(Which no longer generate..) after entering your IMEI, and with these 2 files you would need to 'sign' every app with another program before installing them. (So that the app would know that the installation is approved by you, and it's 'to work' only on your phone, as it has the same IMEI in it!)..