First of all free some disk space to be able to apply all updates to Exchange 2003 and specially SP2!
It will allow you to address more disk space to the exchange store.
Than make backup and verify it.
Open your exhcange server installation folder and locate ESEUTIL.EXE
Reffer to Microsoft's website for exact instructions on how to use it. This is a tool for defragmenting and optimizing the exchange database.
The whole process in the first time you do it should take you almost the whole afternoon. So be prepared to wait for a long time to complete it.
When done MOVE your eschange storage to bigger disk(s). The best practice in Exch2K3 is to move the store away from the system disk right after you install it.
Be prepared to run eseutil once weekly for the next month and analyze the disk costs in this period. If neccessary, there is a way to allow Exchange Server to address even more disk space (64GB if i remember correct) with a registry fix but nobody suggests it.