my computer is freezing when you start it up. how can i fix it?

i have a medion computer bought from aldi. when i turn it on, i go on to the inter net for things, but it just freeze's. to try and turn it off is hopless, i have to pull the plug out of the socket to get it off. how can i cure this problem? i have come round to the library to use their computer to ask for help.