web pages open up incorrectly

Web pages are opening up with all these symbols, numbers and letters. When I refresh the page, sometimes once, sometimes 2 or 3 times, it usually loads correctly. What is causing this?
Other times it says that the page cannot be found or in firefox it says content encoding error. When I retry it usually opens. This is so annoying and a waste of time. I have run all antivirus, etc. which did not show a problem.
Windows Vista, Toshiba laptop.
Thank you for your assistance.