My fonts are small & almost unreadable.BUT if I make them larger,my desktop overlaps! What can I do to remedy?

When I open my e-mail or play games on here the lettering is so small I have to set close to my screen.I have a new computer with windows 7 & am having many problems due to being used to having my older computer with, simple XP.How do I make the fonts larger & readable without my desktop overlapping the screen? I need someones help very badly. Please explain to me in VERY easy understandable words as I am on seizure medications which have caused short term memory loss & do not understand unless it is explained very plain to me.