Cant open task manager, notepad, or anything commanded in "run"...plz help me..

[boombox24]

so heres the story, my brother was looking at adult sites he shouldnt be lookin at and informed me that "theres something wrong with your computer..." i think ,my pc has a virus or adware or something, i constintly have messages saying my computer is infected and have website pop ups. norton security says theres nothing worong.... but i did some research on how to remove w/e is on my pc and i found out i cant open task manager, notepad, or proccess any commands thru run....can someone help me plz