I'm not sure if it will be something you need a technician to set up,
Not at all, it is fairly simple. At least, the 2 VM programs I have messed with (VMware&VirtualBox) are.
Does Avast work OK with a virtual machine, are there any complications or issues?
Depends on what you mean....inside the VM (Guest) or outside (Host). Installed on a host, sure, it is just as normal. Installed in a guest, you
might possibly run into problems with parts of Avast! that use sandboxing.
The guy in the local PC shop says a VM might help, but that the VM software is expensive to buy, is that so in your opinion?
VirtualBox is free.
He also said it's a bit like having a dual boot, and it can use a separate operating system. Does it have to use it's own OS or is that just an option?
A VM creates a folder on whatever OS you are running. This folder is for all intents and purposes a blank HDD. It does not run its own OS....it needs you to decide what to install on it. In the case of wanting to create a Windows VM, that of course involves purchasing an OS install disc if you do not have an old one lying around.
Its not as efficient (resource-wise) as a dual-boot, since the Guest needs the Host running to stay on.
As to whether they will protect you from all infections, no, not even if you install a Linux VM are you completely safe. The Host and Guest are networked together, and you might like the Shared Folders option when you see it, that lessens the security again some. But it is a very effective additional layer.