When using Gmail, go to
https://gmail.google.com instead of the default
http://gmail.google.com to open a secure session.
In many cases, Norton and McAfee antivirus will actually slow down your computer.
Use a browser such as Firefox or Opera to increase security as well as capability while browsing the internet.
Adobe Reader too slow? Try Foxit PDF reader.
http://www.foxitsoftware.com/pdf/rd_intro.php
If you're looking for more control over your computer, and don't mind a bit of extra work, try linux. You can dual-boot it with Windows so you don't have to give anything up.