making the program is easy part
!
infecting the target system is the main task,achieved by penetrating through layered security depending upon the setup. all the network peeps here would agree how hard it can be to hack in if an industrial strength firewall is configured properly and locked down at a very granular level.
Now the first question should be how the computer was infected? , was it done over the WWW or was the PC physically bugged.
If it was done over WWW how the hacker penetrated stringent defence network.. or is it that the target PC actually visited some malicious honey pot and got infected. (This is very common now a days where a honey pots are setup in DMZ and someone trying to gain access think they hacked in but actually they are only accessing fake server which in turns infects the source.)
Another possibility is where hackers had physical access to the device and they bugged it - so it uploads info to servers whenever user connects to internet.
from my experience i have noticed that most of the times networks / PCs get hacked because proper security is not applied and security policies are not adhered by the end user.