The problem may also relate to your resolution level. If your resolution
decreases (i.e., your display uses fewer pixels of height & width, resulting
in a "coarser" picture), everything on the screen will always look bigger.
Conversely, if you increase resolution (more pixels, resulting in a crisper
picture), everything will look smaller.
ANONYMOUS' suggestion will only change the fonts, not the icon size.
To change the resolution size (in WinXP; I recall that other systems are
similar):
1. Right click on the desktop (that's the typicallly blue field of your
display)
2. Select Properties.
3. Select the Settings Tab.
4. Use the Screen Resolution sliding scale to change resolution. (In your
case, increase resolution to make everything on your display look smaller.
Decreasing resolution will make everything look bigger.)
You may have changed your resolution inadvertently or downloaded an update
which had a similar effect. 18 months ago, I had the opposite problem from
yours with my old Dell Inspiron laptop--everything shrank to miniscule size
after I downloaded a video driver update, even though I had not changed the
resolution settings.