Recently i did some tweak in my vista ultimate. now i can see a strange thing. when i click 'Computer' icon and leave it minimized after a while it shows me a folder icon in taskbar instead of computer icon. so plz guys help me to fix it as default

Try and restore your computer,Or wat the changes you did for tweaking ? visual effects setting etc? or any registry change? i can't suggest you without knowing that,
so better try to restore your pc or cheack whether have u made any changes to themes and visual setting in my computer properties.