hi,is there any software which can resize a png file of particular size into different sizes at a time?For eg: if i want to resize a png file of size 40x40 into these sizes 18x18,64x64,128x128.Thnx in advance.  
 
 

Up-sizing PNGs will make them look pixelized (distorted).

i think so
and i only can say what neil say

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Bad English because German^^

the icons are surely going to lose their quality if you resize them however , the icon still looks good if you decrease it's size to some extent but it gets pixelated if you increase it's size even by 10% , so if you want to increase the size , I would recommend you to remake the icon

Use Photoshop for png-png resizing... Use the save for web feature...
Use Axialis Iconworkshop for png-ico conversions...