My PC gives errors that hint at it running off of a server, but I don't know of any server. What is wrong?

My PC is running very slowly, it takes longer than usual to open programs. Sometimes the programs will take too long and I get a windows error saying that it has timed out. On one occasion, one of my programs didn't even attempt to start up and it only gave me a windows error saying that the server could not handle the request. I know of no server and I only have an internet connection and one other computer also connected to the internet. When I unplug my internet connection it doesn't seem to give the previously mentioned problems. This happened an hour ago and hasn't occured since. What is wrong and how do I fix it?