Access denied...error

I was working with Windows 2000 server, VS.NET 2003 and office 2000. I was developing an ASP.NET application, which dynamically creates an excel file and throws back to user. Everything worked fine....creation...writing...throwing to user in the form of download etc....everything is very fine....

But...I reinstalled my server with Windows 2003 server, VS.NET 2003 and office 2000. The same web application was working fine except the part of Excel file creation...it is absolutely having no problem with directory or file security....I set them quite correct...but still says...Access is denied...to the specific resource...

The error gets raised at the following statement (not even at the time of saving Excel file)..

dim xl as new Excel.application

It is just unable to instantiate the object....and gives the error Access denied....

Can anyone help me please...