After a fresh windows xp install, I unfortunately installed "Vista Icon Pack 3.0". Before that every thing was right. after this, my computer went terribly slow, especially in start up.
after watching task Manager, I found that all my cpu performance is taken by ntvdm.exe application. I clicked "end process" and after that everything ran very very fine and CPU went from 100% to 4%.
For me, ntvdm.exe does not start automatically, but with this, my computer gets tooooooooooooo slow. If its not a virus then may be more dangerous than a virus, because it effects the PC performance more than a virus could do.