I did a system clean-up and updated a load of programs. Following Tarun's advice, I uninstalled Comodo and installed Outpost Security Suite. It works well with pre-installed avast, auto-disabling it's own anti-virus. However, Outpost caused massive surges of CPU usage, with my CPU typically clocking in at 100%. causing frequent freezing and music being played on WMP to be played incorrectly.
After having shutdown Outpost and tested everything on the PC, everything is working fine, so I have now uninstalled Outpost. Can anyone suggest other decent firewalls?