Yes, it is possible someone else can access your computer with VNC. It all depends on how it was set up.
What is your internet access? Is your computer hooked up directly to your dsl or cable modem? If so, then you are very vulnerable to someone accessing your computer using vnc if they know the vnc password or if there is no password for vnc.
Ways to protect your computer from access -
- Put a router with a firewall between your computer and the internet and make sure all ports are blocked and the router can't be accessed from the internet. (Most routers now do that by default so you just have to plug them in with very little set up.)
- run a firewall on your computer itself and make sure all incoming ports are blocked - check it regularly to make sure something hasn't opened ports
The following don't prevent access but make it more likely you will notice any access.
- disconnect your computer from the internet when you aren't using it.
- turn off your computer when not using it.