Every PC I’ve had that came with McAfee was slow and had frequent crashes. Deleting McAfee invariably fixed it.
Between the free versions of Avast, AdAware and Spybot, I’ve had few issues with viruses and adware. It also helps to avoid web sites that push adware and to have sense enough not to click on unknown links and attachments.
Just for s—ts and giggles, open up Windows Task Manager and check the CPU usage of the processes, McAfee runs about 3 or 4 processes and they can easily take up at least 50% of your CPU.