First, I am glad that igrid is able to support unicode

However, I have an issue with some xp computers which would not display chinese unicode characters

No problem with this in win7 or win8

The strange thing is that the xp computer which could not display the chinese characters in igrid however, are able to display the chinese characters in MS Excel.

I am quite puzzled why this happens. Do have any ideas?

Most likely, the problem is outside of iGrid. The control uses the system fonts, but to display Chinese characters in Windows XP, you need to activate this option too. I have just googled 'windows xp chinese support' and found a lot of resources about it, for instance:

http://www.chinese-tools...esources/windows-xp.html 
https://www.microsoft.co..._languages.mspx?mfr=true 

Microsoft Office may use its own set of fonts and display system so it does not require this feature to be installed.

Try to install the East Asian languages support as described in the links and play with different fonts in iGrid. If possible, post the results of your experiments here.