That’s a bit weird. I’m running BC on XP SP3 and until not very long ago I was using a SpeedTouch router and never experienced such a thing. This is not to say that your issue isn’t real, just that on a similar configuration I didn’t experience such issues.
I don’t mean to put you down even more than you are but Zone Alarm has had a long history of “not playing nice” with many apps, BitTorrent clients included. At least in some previous versions we used to test it didn’t unload the executable and still filtered connections even when you disabled it.
Now I’m not saying that it is the cause of your issues but just to be on the safe side, you’ll need to uninstall it for testing purposes as we can’t confirm if the newer versions of ZA display the same above-mentioned behavior or not.
For most users, in fact, the Windows firewall will do just fine (and BC can configure it automatically) but if you need a third-party firewall there are better options out there. Check this site for a top of firewalls.
Now back to your issue. If your Eset installation includes a Internet security module (i.e. firewall or anything alike) make sure you disable it too, for testing sake.
Then go in BC Options–>BitTorrent and disable DHT. Also disable LT-Seeding on the Options–>LT-Seeding page. See if this helps bring in line the browser responsiveness issues you were mentioning.
As for the system crash, does this happen every time you leave BC running for a longer time (i.e. can you confirm that it is in fact BC who’s crashing your system)? We don’t have, so far, other reports of crashes caused by v.1.27 so this is not a known or frequently encountered issue/bug.
Nonetheless, if you see this happening repeatedly try downgrading to a previous version (e.g. v.1.26 or lower) and see if the same thing happens.
Then number of tasks you’re running doesn’t have a bearing on the stability of the client (I’m currently running 245 of active tasks with no problem whatsoever, on a 4 years old laptop with only 2GB RAM, on XP SP3 in v.1.27); at best it may slow down your system a bit a times during some operations and depending on the “power” of your PC.
Besides, BC couldn’t have touched your firewall settings since it doesn’t even attempt to configure third-party firewalls; this was most probably a result of ZA’s poor handling of your system’s crash and is at best a collateral damage (provided BC is indeed the cause of your system crash). Anyway, a serious firewall shouldn’t loose its settings following a system crash, unless the settings were made in that very session and following the crash the Windows registry got corrupted and reverted back to a previous state or Windows didn’t manage to save the newly made settings (I don’t even know if ZA uses the registry to store configuration settings but it seems so from what you say).