Kaspersky is one of the better programs, with few issues that slow things down.
What does interfer is when you have multiple programs.
If you use Kaspersky, uninstall all other antivirus programs.
Then have only two antispyware programs, such as Adaware 2008, MalwareBytes, or SuperAntispyware, Windows Defender, Malicous Software Tool, etc. but not more than three. One anitvirus, and two spyware programs, properly selected, should not slow you down, and should give you the protection you need.
We have found we prefer to pay for Spyware Doctor 5.5 or Webroot's antispyware, rather than use a bunch of free ones that change frequently as they experiment with us.