My entire desktop is crashed,alihave is taskm

Hello,
recently my computer came down with something and my entire desktop no longer shows up, no progam icons, no windows taks bar, nothing, when i turn the computer on an antivirus progam pops up and starts saying i have 500+ virus's, i know this to be the virus, then it shuts down and my screen is blank except for the back picture. I can run the taskmanager and eventually get the internet going with "run new program " and get the comand prompt working but thats about it
plz help