Unfortunately there are many attack vectors against wallet software, and a hardware wallet might be the best solution in terms of ease of use and enhanced security.
If your computer is infected maybe you downloaded a fake Daedalus while still visiting the correct link. If you still have the installer you can validate this by following the instructions from the Daedalus website. I’ll paste it here, as if your computer is infected it may not display the correct checksums.
This is just one step in figuring out what happened.
Of course there are many more attack possibilities like key loggers, dictionary brute force attacks against weak spending passwords, etc.