Is it just when you ae browsing the internet? if so hold down the CTRL key and use the + and - keys to adjust the size.
If it's everything on your computer then the screen resolution has probably been changed.
For XP
http://www.microsoft....olution.mspx?mfr=true
For Vista
http://windows.micros...nge-screen-resolution
For windows7
http://windows.micros...our-screen-resolution
(in future please include at least the version of windows you are using to assist in answering your questions)