When you first move into your new apartment it's imperative to change the locks to your external openings, doors and any windows. You can't be sure who previously has had a set of keys, and i'm sure you wouldn't like an uninvited intruder to just walk in. At this point let me say that any external opening to the outside should have a lock that can be locked from the inside with a key. A keyed deadbolt or the like. This hinders the thief who would make off with your larger objects such as sound system or plasma screen, by making it difficult to get these things out on mass. So a security system works in two ways. Firstly by trying to stop an intruder and making him pass on to an easier target,but in the case of entry being gained hindering them from ease of exit.
Back to changing the locks. If you are a bit handy and have somebody to stay in the house, i would suggest taking off the exiting locks and taking them down to a locksmith and have them change your barrels in the locks. Try and get them all keyed alike, so you don't have the problem of finding different keys when you wish to open the windows. Sometimes if the locks are of different brands they can't be keyed alike. Taking off a lock isn't that difficult, just take note how it came off so you can put it back later. By taking the locks to a locksmith you will save on a callout fee and of course the labour of taking the locks off and putting back on.
Now if you are fairly handy you can get yourself down to the home depot and pick up the re-pin lock sets that are available to most makes of locks. Of course a call to the store before you charge off there is good sense, to make sure they have the brand you'll need, but there shouldn't be too much trouble with kwikset or schlage, they cost around $10 a set. Re-pinning the locks yourself isn't that difficult a job. Take the lock off from the door and pull it apart and remove the lock core, take out the tumbler and then put the new pins in. They are color coded so it's easy. The pins will be all flush when you place the new key in the tumbler. Ensuring you have done a correct job. Re-assemble the lock, put it back on the door and job done at around 10% of the locksmith's cost.