getting error message on boot up

i have received a cpu from a friend,worked fine. Currently running windows xp pro. out of nowhere i turned it on only to have it stop in the middle of booting and give me this message

“STOP: c0000135 (unable to locate componet)
This application failed to start because USER.dll was not found. reinstalling the application may fix this problem.”

i didnt download/install anything new is connected wirelessly thru dsl,,,

it is for my kids to use, may have gotten a virus…

PLEASE HELP!!!!!!!!