It should all work fine, since Xvirus products do not interfere with other security products (no device drivers, injection and hooking, etc.). As for VoodoShield, all it does is monitor process execution and send this information back between user-mode processes and the device driver (which receives the process execution requests via a callback notification) for scanning purposes, so it should work fine alongside Avast Free Anti-Virus and Xvirus Firewall.
I recommend you always have an up-to-date system image for backup purposes, regardless, use system restore (make sure it's enabled - vendors like Avast create one automatically for you in the background during the installation progress).