What you'll want to do is go to the NVIDIA Control Panel within your computer's Control Panel. There you must click "Customize".
From that window you select "Create custom resolution". You'll probably have to select a multiple of 24hz, like 48hz, 72hz (72hz is the only one I can run) as monitors with "true" 24hz are quite uncommon.
Hope this helps.
OS: Windows 7 Ultimate 64-bit (6.1, Build 7601) Service Pack 1
Processor: Intel(R) Core(TM)2 CPU 6420 @ 2.13GHz (2 CPUs), ~2.1GHz
Memory: 2048MB RAM
DirectX Version: DirectX 11
Video card name: NVIDIA GeForce 7900 GS
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