3cx stores most of its data in the Program Data folder under v2 profiles and under app data in v1 (yours). Although there are some parameters that can be changed it wont help you and wouldnt recommend it.
Recordings can be placed on a different server and/or drive which helps. This is under Settings > General and the Global Options tab. You will need to set the necessary permissions on the folder/drive you move to.
I would recommend moving your recordings to your new D drive and leave everything else as those files will be small compared to the recordings.
Given that you are also an IT Manager, I would assume that you would also have access to a server in your department. I recommend running 3cx on that instead of a windows pc. It does not need many resources and is a lot better at resource management than XP/Vista/7. Even running on a VM gives great performance.