...especially at startup. The more items you have running, the more memory/resources you have used.. Uninstall those that you don't need. Try shutting down unnecessary startup programs in "Services" and possibly "msconfig". Unfortunately, there are lots of things that could be causing the issue..

Although AVG isn't a bad antivirus program, it won't find everything.. Try downloading, installing, updating, and running full scans using both of the free malware scanning tools below. Delete anything they find.:

ADWCleaner
http://www.bleepingcomputer.com/download/adwcleaner/

Malwarebytes
http://www.bleepingcomputer.com/download/malwarebytes-anti-malware/

Hope this helps.

Grif