1. Common Sense 2.0
2. BACKUPS!
3. BACKUPS!
4. BACKUPS!
5. If you're up for a bit of effort, then you can make the system (not the data like docs, images, etc) bulletproof: Apps exist that can make it so that any changes made to the system are just temporary and discarded on the next boot. With such apps, you could delete half your windows directory, yet on a reboot, you'd see that none of this actually happened.
The way how you'd use such a setup is this: When you get new software, you first install it while the system is in protected mode. If the software turns out evil, or you decide that you do not like it, just reboot, and everything will be back to as it had been at your previous boot. This way, you could "test drive" software, and it would be technically impossible for it to do any damage to the system, because nothing that happens during this "session" is permanent. when you then decide that you trust the software, you reboot into unprotected mode, install the software again (this time its permanent because the system is in unprotected mode), and then reboot again into protected mode.
If you're willing to put up with the additional effort of such a setup, you dont need any resident antimalware app at all - because while in protected mode, nothing that happens to the system is permanent - the system simply cannot change across reboots. A non-resident on-demand malware scanner may however be useful to check your pics, docs, etc one per week or so.
6. Install a resident anti-malware software again, see your system go to a crawl, be annoyed by false positives, and disappointed again when you learn once more, that it is technically impossible for malware to be detected reliably. I can write you a batchfile in 3mins that will wreak havoc on your system, and no malware scanner will notice it - because: A malware scanner either needs to be told exactly what to match, or it needs to guess via heuristics.... but for things like file-deletes and so on, it is not possible for a machine to know if you actually want this to happen, or if its against your wishes.