Why my desktop icons go extra large everytime?

Hi guys, I have windows XP but don't know which operating system in particular like 2000 or 2003.So I have a problem with it that every time in few days or take it as once in a week the desktop icons go bold and extra large without making any changes when opened any any files and documents even they appear fluffy kind and large is it due to virus or what please help me to fix this.
