If you're connected to the Internet with an always-on broadband connection, your PC is being scanned by botnets many times every hour of the day.
If your PC is not updated with the latest patch from Microsoft, your PC can be infected with virus within 3 hours. You don't even need to open a browser anymore to be infected.
If you're on a corporate LAN that doesn't have a good edge defense & end-point security, it can be infected the minute it's hooked onto the LAN.
Yes, MS Defender is ok against viruses from thumbdrive & email attachment but it's woefully inadequate against all the possible attack vectors from malwares.
The worst thing that can happen is a stealthy virus that install a rootkit & keylogger or a backdoor that monitor all your activities including your passwords, NRIC, bank account, phone numbers, etc. & send all your private info to a criminal syndicate.
Win 8 is pretty secure but all the hacker need is one vulnerability to be able to control your PC.