I have seen this problem with several computers and each time have identified the cause as being hardware related. Here are some of the things I have done to fix the problem.
1. Computer would run all day but crash under FS2000. Solution, install additional fan in the case to keep graphic card operating at cooler temps. These cards will get hot depending on the demand softwares demand.
2. AGP card was not seated correctly. Card would run fine nearly all day giving no hint of a problem, and then freeze with only the mouse pointer moving on the screen.
3. AGP card had the fan facing a hot PCI card with little space between the PCI card and the AGP card such that the AGP fan was struggling to keep the card cool. Moved the PCI card to another slot and the problem went immediately.
4. The computer would run fine all day and then give up for no reason. Have solved this problem by only running a single memory bank, or moving the bank to another slot. Most time, the problem is when failing memory is under high demand it just gives up. Replacing the memory can solve the problem.