Windows XP hangs and is slow

All the computers at my work place which have windows xp are very slow to start especially those on the network after pressing the ctrl+alt+Del buttons and entering the necessary password it takes about 7 minutes for it to bring the desktop screen, other operating systems are fast. Why is it so? It is also slow when i try to open or close a file in any program even windows 98 is faster. secondly the computer keeps on hanging unecessarily and asks me to send the errors to microsoft,even on something small like saving it hangs and it is slow, why?