For the last 2 days my antivirus programs have not been working. I keep getting a message: UI failed to load - restart service. But restarting comes up with the same message. I've tried Avast, didn't work so I uninstalled it and tried AVG. Same result, exactly the same message. Could some virus have gotten into the computer and stopped any AV working?

Absolutely. Download the AV removal programs for AV's you have used to remove any possible broken programs. I would also recommend running the free ESET online scanner set to remove any possible malicious programs and let it run to completion. Not any guarantees but that should get your computer in better shape to work on. Once it will install and run a AV you are on the right track. I would highly recommend staying away from AVG. When it gets broken it is harder to get rid of than the viruses. If you have access to another computer you could install your hard drive in as a secondary the Antivirus scan will be a lot more effective because you will be running from an Operating System that hasn't been hooked. A virus usually tries to disable your ability to remove it.

Many thanks again, Falcon, that did the trick, AV working again.