Nothing's hack-proof. Windows is most widely used therefore it's the one hackers target. Had Apple been the one to be on top it would be in the same spot.
While nothing is 100% secure, earlier Windows machines had holes so big you could drive a Mac truck through them.
Windows 98 and to a lesser degree Win2k machines had many critical ports sitting on the Internet side open BY DEFAULT... Some were very difficult to close. And even then with the port closed it was a big red flag on the Internet that there was a machine at that IP address which made it a target.
And that’s not including early Internet Explorer and OutLook Express problems. Where early on security was simply not considered at all - literally.
That’s not true of most other OS.
Before Vista something could install itself without your knowledge. Play what you think is an audio CD and get a nasty rootkit (or anything else) installed on you machine that may not be even possible to remove...
Again, security not considered with the autorun “feature”.
Other OS’s managed to avoid this problem.
Hacker code can be hidden in countless locations on a Windows machine. Trying to find it can be very difficult. Again, other OS’s don’t allow things to be buried all over.
Windows was built promiscuous by design. The vision was that of an ultra friendly machine which would do what you wanted it to do with all the data all the time. If everybody played nice, that would be an ideal operating system, just like it would be ideal not to have to worry about locking your doors or needing keys to start your car.