You might want to wait a bit to apply AutoCAD 2013 SP1. I have not had any problems so far myself though.
Fatal error after applying Service Pack 1 for AutoCAD 2013 and AutoCAD LT 2013
You are encountering this issue due to a problem with Service Pack 1 for AutoCAD 2013 and AutoCAD LT 2013. We are actively working on resolving this issue. It is recommended that you uninstall Service Pack 1 until further notice. TS20364390
UPDATE: AutoCAD 2013 Service Pack 1.1
