I am running Windows XP>
If you look at the bottom right corner of your homepage you should see a little % symbol.(Mine is at 100%)
If you click on the down arrow next to it it gives you options from 50% to 400%.
It might be that you have clicked this by mistake,if you click on the 100% it should restore your screen to the original setting.
If this doesn't work, then someone else will have to make another suggestion!