turning off

I use the computer during normal downloading the computer automatically switches off as though i just unplugged it I changed all the memorys hardrives etc and the drivers are all updated and no explanation marks by them I also have alot of problems like it is very low on memory but has 1 GB 400 MHZ memory sticks a pentium 4 3.2 GHZ cpu with an a-BIT lga 775 motherboard. I changed the mother board aswell seems to run a little better but not fully. I even reved programs down to just windows and no better. The other I thing is the floppy disk drive did not work in an amd processor board 754 and the elite groupe 775 board which does the same problem but worked in an a-bit
if any one could help I would be grateful

many thanks

jason hawkins