Branwyn
My system is Win XP Pro 64bit and I have used Avast free since I built it roughly a couple of years ago. I've upgraded as they came along ( now on 5.0.594) and have had no problems except a slowing of Avast at startup.
I use Windows firewall, Superantispyware Pro and Malwarebytes. Preview pane in OE turned off and that's it, touch wood never had a problem.
PS
I really just joined the forum to thank everyone for such a great free Anti Virus programme.
AMD Phenom Quad 2.3mhz, 8gb ram, Win XP Pro x64bit, 50mb broadband.