A shop counter can test your Ethernet port pretty fast. I'll share how I do this.

I boot up Puppy or Mint Linux from USB or DVD as this sidesteps Windows software issues without touching your current files or install.

This is all over the web and free. It's always good to keep this around for test and backup.
NO LINUX SKILLS REQUIRED: Read this old advice I gave at http://tips.oncomputers.info/archives2004/0401/2004-Jan-11.htm
Bob