Camilo, go into Task Manager to the Processes tab and find how much memory the SLDWORKS.exe is using. Either you have a memory leak-for which the answer is usually to reboot your machine-or your models are too big and complex for your computer.
You should also run a SolidWorks Rx report and post the results up here.
One thing that might be affecting the stability of your system is your video card. The Geforce series cards will work with SolidWorks OK, but their drivers don't have the full OpenGL implementation that SolidWorks needs in order to render the model and the UI. Most people who run GeForce cards can get them to work to some extent, so it's unlikely to be the root cause of your problem.