In a country with very high unemployment rate and a great deal of financial inequality, crime of opportunity does occur. However by following some simple precautions and being aware of your surroundings you should easily be able to minimise your chances of being a victim.
Be careful in major centres and never leave anything in an unattended vehicle in any city or town. Pick pocketing occurs in the towns and cities but if you exercise some caution and do not carry huge day packs or camera bags around with you the risk is minimised.
Muggings, although becoming more common, almost never include violence, do not resist if anyone points a knife at you (unless you have that black belt in karate). If you're mugged at gun point consider yourself special as this is almost unheard of.
Windhoek now has two police forces the municipally funded City Police & the state operated police force. If you are in need of assistance it is generally best to contact the city police as their response times are excellent and they are generally extremely helpful.
In national parks such as Etosha or Sossusvlei crime is rare, it is generally safe to leave your luggage in your car, accommodation establishment or even tent just make sure your car is locked at all times.
Trust local advice, and don't be afraid to ask people at your accommodation establishment, but it is worth remembering that many southern Africans are rather paranoid when it comes to crime and love to regale visitors with horror stories.
"Crime does not pay ... as well as politics.", Alfred E. Newman