Defference betwween Windows98 & WindowsXP .What are the main difference between these two.

I'm sorry, but according to your signature you are a Sys Admin, surely this information should be well known to you already!
Also a simple Google search will give you this information in about 5 seconds.

Microsoft will tell you that XP has improved security, faster log in's, better performance, better GUI and Other sort of stuff but my opinion points to the fact that nothing much changes systems wise only improvements on GUI.

Take Curlyben's advice and use Google!

Surprise's me that he's a network admin! Google it

I agree with Ben, this is a strange question to be asked from someone claiming to be a Systems Admin.

But I will answer that Win 98 is based on the old DOS kernal, while XP is based on the NT kernal. That is the major difference.