Have you been trying out any 3rd party graphics tools or custom themes for the desktop? The registry values for the default Windows should look like these. You wiill first have to type regedit at the Run prompt and press enter to open up the editor for the system registry. From there open up the following registry hive.
HKEY_CURRENT_USER\Control Panel\desktop\WindowMetrics
"Shell Icon BPP"="16"
"Shell Icon Size"="32"