before parrot answer to this thread: DON'T BUY A MAC
if you want to, go to your local mac dealer and buy one. point. stop reading here
otherwise, the essential parts of a pc are:
1. case with power supply unit. depending on the components you are looking forward to buy, the capacity of the power supply unit shouldn't be less than 350 Watt, better 400 Watt
2. mainboard
3. graficadapter
4. cpu with heatsink
5. ram (for win xp 1 gig is recommended)
6. harddisk
7. dvd-rom or dvd-writer and
8. floppy-disk (optional, as most operating systems nowadays come on cd or dvd).
as optional components there will be
9. sound adapter (most motherboards today have a built-in one)
10. lan adapter (same as sound adapter). if you're using dsl as your internet connection, you definetely must have one.
11. dial-up-adapter, for work if needed, like modem or isdn-adapter
12. irda (infrared) or bluetooth adapter for connecting mobile phones and the like
13. wireless lan adapter, if you are looking forward to connect several pc's in different rooms to the internet.
if you choose 13., you also need a wireless lan access point.
there will be no recommendations on the several parts from me, as i
1. don't know how much money you would spend
2. don't know what usage the pc is intended for
3. hate flame threads

(intel vs. amd, nvidia vs. ati etc. etc. etc.).
if you want some recommendations for puchase, pm me.