NT – Data Error cyclic redundancy

Running NT 4 SP6, dual 500s w/1 gig ram, box is having major issues. If left on overnight, is frozen next morning.
User recieved error message “Missing Operating System”.
I benched it and booted up fine, I installed NAV Corporate, ran check, no virus.
I am to run SP6 again and I keep getting the error: Cannot copy sp6i386: Data error (cyclic redundancy check)