Hi Bonneville:
System Specs would be most helpful. Is it a laptop or a desktop, Brand, Model, Processor, RAM, Hard Drive capacity, etc.
Are you running a hard-wired keyboard and a hard-wired mouse or is one or both wireless? If you are on wireless, try to borrow a hard-wired usb keyboard and hard-wired mouse and try with those. If the problem disappears, it could be low batteries on your wireless keyboard or mouse or both(have you replaced the batteries in those??). Replace all batteries in one or both devices and try again.
If you do have both keyboard and mouse that are wireless, such as the logitech or microsoft combo units, and the new batteries trick doesn't fix it--I'm going to guess that the 8.1 installed some new drivers that are not compatible with your hardware. You should Google *MICROSOFT UPGRADE ASSISTANT*, download and run that program. You may find that your wireless keyboard/mouse drivers need to be updated. Download and install drivers from Manufacturer's website (Microsoft, Logitech, Kensington, etc.) and re-try.
If this doesn't work; or you visit the websites and they don't yet have 8.1 drivers available for your keyboard/mouse model, be PATIENT! It usually takes several weeks or even months for the Manufacturers to catch up on the thousands of models of their devices they have sold over the last 5 years and update all those drivers. Essentially, this problem you have will be repeated for people using older printers, scanners, digital cameras, etc. after the 8.1 update.
BIGBEARJEDI