right click on desktop, choose properties,
choose settings tab
click show all modes, choose the screen size you want & the depth of colour
ie.800 by 600, True Color (32 bit), 60 Hertz
or in your case, 1024 by 768, True Color (32bit), 60 Hertz
if you have problems with these settings, try reducing the colour depth ie 24Bit or even down to 16Bit.
apply and ok, wait and your screen will change size, you have 15 seconds to choose to accept or reject the settings, if the screen goes funny and you cant see anything DO NOT PRESS ANYTHING, your screen will reset to its original, if you do press by accident and accept you will find ithard to get it back so be carefull.