what is happening is that your computer is generating a fatal system error. This can be due to a number of things. The fact that you only have 128MB of ram isnt likely the cause, but the RAM could be going south. Also, if the ram is overclocked or set to a higher CAS rating than it is meant to be, you could possibly be getting these errors. That being said, your RAM may very well not be the cause. What you can do to try and troubleshoot the problem is the following...
open up your System in the control panel and select Advanced. Choose startup and recovery. Under the System Failure setting, you can see that the checkbox for auto reboot is what is causing your computer to reboot. Leave those settings alone, but change the one that says Write Debugging Information and choose Complete Memory Dump. As you have 128MB of Ram on your computer, make sure you have a pagefile on your boot partition that is at least 129MB large. You can also see where the Dump file will be created. Next time it happens, view this file and hopefully it can shed some light on your problem. Like i said, the cause can be many things. Good luck.