virtualbox.org

I've read a couple of posts about ways that a virus can move from a hosted OS to the hosting OS, and was wondering if someone could clarify. In order to get a "shared folder" in the guest OS, I would have to go in and manually set it as shared correct? I am just unclear on what a shared folder is...I've never used one before.
Also mentioned were possible spreading over the network. Would XP straight out of the box be a fairly secure system in this case?
Also, I was wondering if I enable USB on the hosted OS, would this be a further security risk?

Set shared folder: Yes, you need to do this manually. It's only available after you have the Guest Addtitions installed. Normal network shares are just that, network shares.

XP out of the box: Very buggy and just plain insecure. If you want a virus, use that without any updates or antivirus application and browse the internet. Any infected site will immediatly infect your XP.

USB risk: This depends. Would the Host use it? Does it have a proper antivirus that is up2date? Do you use it in other systems that don't have any security software installed?

If you look at risks, there is no way to be 100% safe as long as there is a possibility to use external media. Either network, floppy, cd/dvd rom or USB, they all pose a risk if a virus is on it. So if you want to make sure your system won't be compromised, don't use it .