All your questions have been answered on the forum, but here goes:
1. When the VM is not actively running, it uses no resources. Even when "suspended." Which is how I leave mine (as opposed to "powered off") all the time. Has no impact on Mac performance when not running, only when the VM session is active.
2. It is very easy to delete the virtual machines - you literally just drag the relevant folder to the trash. Couldn't be simpler. Then just uninstall Parallels or Fusion, as the case may be.
3. As a Windows "machine," your VM will be subject to the same risk of viruses and malware as any other Windows machine, so you should install an antivirus program (and whatever else you want) - I use AVG 8.0 free antivirus, but there are numerous choices. And no, a virus on your VM will not affect your Mac OS. And, what's more, if you take regular "snapshots" of your VM, and you wind up getting a virus, you just restore back to a previous snapshot and you're all set. Cool, huh?
I've used Windows machines for years, but it is now abundantly clear to me that the safest, easiest way to run a Windows machine is actually to run it as a VM on a Mac under Fusion (or Parallels). Safer, simpler to run, easier to configure, better security, easier to restore, etc. Fast, too. Unless you absolutely need native hardware support (in which case Boot Camp is the better solution), I think you'll find the VM option very useful.
Cheers