For Windows and Linux you best reading is 100% memory use as memory beats hard disk access every time.
But the restarts may point to maintenance issues. Are you keeping up with cleaning with the canned air? For desktops I like to get this done every few months. You?
As to RAM, use Crucial.com and tell which OS. If you are running a 32 bit OS, 2GB is fine.
My pc configuration
processor-intel pentium 2nd generation 2.6 GHz
when I open my browser and start browsing cpu useage goes upto 80% ram upto 90%
sometimes my pc restarts
if I change my mainboard with gigabyte 81 series motherboard and take 4gb ram of 1600 bus...how will be the effect?plz help me