There are a few possibilities to take into account.
1. Just because the tests all pass in Windows doesn't mean that there isn't a problem. As a general rule I prefer to test in DOS due to the fact that you have all the resources available for testing and don't have to worry that perhaps the OS or drivers are standing in the way. If Windows has allocated or locked the damaged/defective areas of the RAM or HDD then our software may not be able to test the problem areas.
2. SC 7.5 is now just about 3 years old. While it is still viable for most systems made since then the tests may be unable to consider the variables introduced by new technology.
3. It is also possible that the HDD and RAM are not at fault. If you have already tested the system with a new HDD and RAM and the tests are still failing, there is always the possibility that the problem is with the Motherboard.
If you can, please provide us with additional information such as what tests you are seeing fail, more information about the system, and any other details you can think to provide.