You probably have to many applications running, click CTRL ALT and DELETE to bring up the task managar to see whats running then click on Processes to see what application or module is eating your resourses, I have no problems with this site or facebook.
Also go to http://www.speedtest.net/ and see what speed your ISP is providing you.
You could be short on HDD space as that often slows down a computer, Perhaps a defrag will help?
I,ve never tried AVG but if its anything like Norton then it will take up a lot of resources then you will be better off with using Zone Alarm Pro or Zone Alarm Seurity suite which are firewalls with anti virus and spyware protection.
Another good site to check your firewall is doing its job is www.grc.com and click on shields up.
To check Pc Performance go to www.pcpitstop.com and run there tests
I hope I have been of some help and assistance.
Paul