Sql server 2008 r2 Reporting server issue

Hi

I am not able to start ssrs service on network service account however it was working fine when i intially configured on network service.I tried in various ways to resolve the issue but its working on domain account not on network service account howeverit needs to be running on network service account

1.when i changed service account from domain account to buit in account(network service),the ssrs service was stopped and disable all the configuration tabs except service account and server start/stop tab
2.i did not backup the encryption key however when i tried to delete encryption key after changed from domain account to service account(network service),i was not able to do it as server  was stopped and all the tabs were disabled.

Could you please find the attachment of error and help me to resolve the issue
ReportServerService--09-22-2012-.log