Don't take a rental car, never - it's a zoo!:
-long pile-ups on the autobahn and near the fair!
-you might end up getting a parking place some miles away

The (special) trains are good and punctual. There are trains from Hamburg and elsewhere direct (and only!) to the fair.
BTW, the weather can be lousy: snow mixed with rain or rain mixed with snow Have fun!

Check this:

Accommodations
The Hannover Fairs USA Special Services Department provides American exhibitors and visitors with a wide variety of accommodations to chose from that fit any budget! Room availability during CeBIT is often limited due to high demand, so we encourage all of our clients to book rooms well in advance.

http://www.hfusa.com/cebit/

Choose from one of these attractively priced rail travel packages for your trip to CeBIT 2004(departure from any station in Germany).

Return rail fare on all regular Deutsche Bahn connections, incl. ICE connections, from any German railway station
One night’s accommodation in a single-bed room in the booked category (breakfast included)
http://www.cebit.de/33624

Special Train Stops during CeBIT 2004
Daily ICE-stops on the north-south distance. The trains start for example in Hamburg, Bremen, Munich or Karlsruhe.
Special trains from west and east. The trains run for example from Berlin via Magdeburg or from Cologne via Bielefeld.
You reach the entrance WEST 1, hall 13, via the Skywalk
http://www.cebit.de/25198_1