Signature couldn't be verified problem with updating

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http://askubuntu.com – I wanted to install LAMP, but didn't install it properly somehow. So trying to remove previous package, I used purge. A large number of package got uninstalled like software centre etc. When I rebooted my system it was unable to login to my desktop. Something like running in low graphic mode appeared. I installed GNOME desktop. I was able to login to GNOME desktop then, but there was no end to my problem after that. I am unable to update now and several package are getting installed with errors. W: GPG error: "http://dl.google.com" stable Release: The following signatures couldn't be verifi (HowTos)