Good catch! Unfortunately these types of malware trojans are all too common. I think everyone has been affected by something like this at one point or another, whether you want to admit it or not.

And if you haven't been affected, then you simply haven't been adventurous enough. :-)

Be careful and scan your computer regularly, and notice things like this, and you'll be fine. As long as you're not holding a large amount of value you're fine.

You're really lucky you noticed. Remember to always be vigilant against viruses. You never know when you can get one. I got One before but luckily I caught it.

it is true that double checking and a safe environment (and backups do not hurt) are important. not everyone does that and they only start when they lose money. the sad part is that for every person who manage to notice this there are probably two who do not and lose some coins.   

good that you noticed early ,  now days there are lot of malware's , trozan and people are trying to hack which will try to gather details.It is better always keep the PC's secured with security feature like antivirus etc , and using strong passwords .

It is not luck that you notice something like this.. You should always and I mean always double check to see if the address is still the same.

I recommend using a hardware wallet like a trezor in this case, keeps you safe from bitcoin and also forces you to double check the address on the trezor itself. So even if hardware changes the address, you will always see the receiving address on the device itself and can confirm if it is where you want to send your coins to or not.

you are right,anti virus is really needed nowadays,if it's about money,i really encourage to pay for premium anti virus for the security and safety
it's better paying off the antivirus company for a little money rather than getting hacked than suddenly become poor,it's digital world that criminality is everywhere

Now the question is, what did you do or download in order to have this trojan installed? Would be helpful to sort out sources of these things.

Good antivirus is not easy to find, nowadays almost any antivirus company claim, that its antivirus is the best. But I think using Kaspersky is the best option, and especially if you use also Anti Malware soft. Of course there are more good solutions, but most often expensive ones, not for everyday user. The main advice - never switch off Kaspersky, it can cause a lot of problems then.  

I experienced  the same scenario when im trying to transafer funds to another  address luckily ive noticed it right away because i have a  habit  on double check or even triple  the address before i send thats why it saves me from that. There are really malwares that cant be detected by AV.