do you get any errors on screen or does it just freeze?
have you installed any new software lately?
graphics cards have a gpu(graphics processor)on them and when its asked to work hard it will heat up,this is normal.
it may become quite warm to the touch.shouldnt burn or anything though.
have a clean out of any software you dont need,defrag your hard drive.
update drivers for motherboard,graphics card etc.
install more memory if you can.