This is not right .... Entropy may, can and will decrease for any closed system. Problem is likelihood of decrease. There is
astronomically small chance that Entropy will decease but it will if you wait for sufficient time.
You might try XP 64 bit edition. It will support more RAM. Yes Power supply means SMPS. Though 4 GB is enough for most purposes , be sure to buy the RAM that matches your MB's FSB .
Try some cisco press books. Though they are cisco specific but will give you a lot of practical knowledge that Tannenbaum would not be able to.... Best of Luck for exam.