Unable To Access Taskmanager Or Change Desktop Background?

i recently had a couple of trojans on my computer. they started to make my computer crash alot and they changed my background to a huge warning sign..it had said warning your computer is infested with many trojans malware and adware. i ran alot of scans and now i am very sure that its all cleaned up. But when i try to access my taskmanager i cannot. i have read another thread wich told me to type in taskmngr.exe or something like that the run thing but it just said task manager has been disabled by admin. how to reenable it and i am the admin. also when i try to change my background via right click>properties>desktop where i should be able to scroll through the backgrounds is all locked up and disabled.