Without more information, all I can do is guess. You'll have to try to test things on your end to see what might be causing it. First, I'd suggest installing a clean copy of Invision (not one you've made changes to) and see if you have the same problems. *You can change settings, just don't edit the script. That will rule out any bugs caused by changes you've made. Also, try with no scripts installed other than Invision. You can always run it as a second copy if you have scripts or anything running that you don't want disabled while you're testing.
I'd also suggest checking your logs and see if you notice anything unusual. See when it crashed and check all logs including status to see if any errors appear or if anyone sent you a message or /ctcp or anything like that.
If you have other programs running (such as a web browser), are those also closed? It's possible the computer is restarting in that case.
You can check your event viewer to see if anything stands out. Run eventvwr.msc from the start menu and look under System and Application. Depending on your version of Windows, those may be in a folder. Just expand the folders until you see them. You'd be looking for anything with marked Red. Yellow warnings aren't important for this. Just check the time frame where Invision probably closed. Look at the details of anything that's red. Something mentioning an unexpected shutdown or application hang may be related.
I'd suggest trying without BitDefender running if you just installed that and see what happens, but if it might take hours for it to close, then that's not good to leave yourself with nothing running. If you have other A/V software running, then it's okay to do that, but if that's all you have, I would avoid that. Or, if you choose to try it, then just don't use your web browser or email until you enable it again.
Let me know what you find out with this and we can go from there.
Also, if you find bugs, please let us know. Fixing it yourself is fine if you know how, but it only helps you. If you let us know about any bugs, we can fix it for everyone. It also makes it easier to provide support if you have an unchanged version of the script.
You can find me at #Invision on irc.irchighway.net or #OldGames on irc.undernet.org if you need me for anything.