You could try clearing your Temp folder (maybe Unity has crashed one time, and left an unwriteable file behind in the Temp folder so you cannot overwrite it).

Otherwise it seems like you have either no read and/or no write rights for one or both of these folders. Try logging in as administrator (if you can) and set the rights correctly - normally you should have at least read rights on C: and write rights on /Temp, but maybe something is different on your machine.