|The phone calls from "Microsoft" are scams. Hang up on them and on no account follow their advice. It will cost you money and/or compromise your security.|
Warnings, and even errors, in the event viewer are quite common and don't necessarily mean that anything is really wrong with your computer. You can easily clear the event logs (just right-click on a log and you'll get an option to clear it), but this is not the cause of any problems you are suffering.
It may be worth using an online virus scanner. I can't recommend a particular one because I have no need for them, and so no experience on which to recommend a particular one. I'm sure someone else can come up with a few suggestions. Do you run any anti-virus/anti-malware software? You certainly should - Microsoft Security Essentials is as good as anything.
If you really are having problems with your computer your best bet may be to take it to a local computer store. Obviously they will charge you to look at it, but they should know what they are doing. And, most important, just hang up on unsolicited phone calls. 90% of them are scams.