crashing

need help ive used all the lastest adware and virus checkers and cleaned this thing out my problem is this my comp keeps freezing up and i mean total freeze no mouse movement no reset no cnt-alt-del- nothing only thing i can do is shut it off by the power button , sometimes is will restart and keep restarting now when i try to start my comp i push the power button and the normal window screen starts and then goes black for exactly 45 sec's to 60 sec's then will start up with the loggin in screen and everythings normal but it didnt used to do this its alwasy taken about 15to 20 seconds to complete start up i did do a scan disc one day and ever since then the slow start up , ok problem two is the freeze up it can do mostly when im on the net but will do it at odd times off the net like when i was running adware it just froze up while running,,ive even restored it back to three months ago when everything was running great any suggestions