There is indeed an option that prevents your computer auto-rebooting when it crashes in sytem properties - but it will not stop your computer crashing if you remove it (it just gives you a better chance to see any error messages such as those shown on the infamous BSOD "Blue Screen Of Death"). BSODs are a notorious sign of bad RAM.
To turn off auto-reboot, please go to System Properties (My Computer->Right Click->Properties) and then choose the Advanced Tab, then Startup and Recovery Settings. If there is a check next to "Automatically Restart" remove it and your system will hang when it crashes, rather than rebooting (most of the time this will work, sometimes the hardware just forces a reboot).
It is possible that the two sticks of RAM you have are just not compatible. Sometimes the RAM will work at lower demand settings (delve into the BIOS there will be some memory settings such as RAS, CAS RAS-CAS - they will all be lumped together). Basically you want to INCREASE these figures to lower demand on the RAM - they refer to the delay between various types of operation in the RAM. The more delay (the higher the number), the more likely the RAM can take what you throw at it. But unfortunately, as I've said, it may just never work
Run memtest
http://www.memtest86.com which only runs from a disk it creates or
http://www.home.earthlink.net/~alegr/ which runs in windows to test your RAM. These will not find ALL problems, but the vast majority of them.
If you're still having trouble, let us know exactly what happens when you get these crashes (if there are any more details), inculding any and all error messages you get.
