The first thing I'd do would be to boot to Safe Mode and check for viruses and other malware. If that doesn't fix it, you can use System Restore to go back to an earlier time.
I have had Windows 8 installed for about 2 weeks. Everything was working fine until yesterday.
A large black rectangle replaced my cursor in some programs. Not in all but especially in Word and when I log in.
I have tried to correct it by resetting my pointer in Mouse, but it is still there.