Download TweakUI from here.
When u'll open it, u'll see an option Repair at bottom of the left side pane. And when u'll select it, u'll get the option to "Repair icons" in right-side pane!
U can also do it manually by deleting "iconcache.db" file (it'll be a HIDDEN file) in following directory:
C:\Documents and Settings\Administrator\Local Settings\Application Data
Here Replace C: with ur system drive and "Administrator" with ur username.