You can skip down to step 2 on that page; it is not necessary to register, if all you want to do is torture test.
2. Run the Prime95.exe file. If the UAC warning appears, click "Yes" to allow it.
3. Select "Just Stress Testing" in the first window that appears.
4. Select "Small FFTs", or whatever tests someone asks you to run. Prime95 will automatically select the maximum number of threads reported by Windows in the "Number of threads to run" box.
5. Keep an eye on your temperatures, and stop the test if the machine starts to overheat.
6. Leave the test running for up to 2 hours, unless errors occur sooner. Errors will show up by one or more of the workers stopping.
As with all hardware tests, Prime95 is not fail-safe. If the test passes for 2 hours with no errors, it is most likely to be stable. Unfortunately, it sometimes makes mistakes and gives a clean bill of health to faulty hardware.
I have no idea what round-off checking is. Is it enabled by default? When I torture tested my CPU for my overclock, I didn't mess with that at all. I knew it had failed because one of the cores suddenly stopped testing and reported errors.
System Manufacturer/Model Number Home built OS Windows 7 Ultimate 32 bit CPU Intel(R) Pentium(R) 4 CPU 3.00GHz Motherboard ASUS P4P800-VM Motherboard Chipset: Intel 865G + ICH5 Memory 2.50 GB RAM Graphics Card NVIDIA GeForce 7600 GS Sound Card SoundMax Integrated Digital Audio (Chip) Monitor(s) Displays ViewSonic VX 1962 wm Screen Resolution 1680 X 1050
Keyboard Microsoft Comfort Curve Keyboard 2000 v10 USB Mouse Logitec optic USB Cooling Fan based Hard Drives Seagate Barracuda 7200.10 80 GB
ST380215A ATA Device 18.6 GB
Western Digital "My Book" external hard drive 750 GB Internet Speed 3.01 Mb/s download 0.64 Mb/s upload