Excel2003 to Excel2007 issues

I have been working on this VB/Excel application. The issue i am facing is that the code which pops up number of excel sheets from VB is working absolutely fine with excel2003 and earlier versions but not with excel 2007.
Please help me fix this issue.

Thanks,
Divya

Its a huge application which pulls in data from access database, invokes excel and populates it.
It then creates many sheets, uses the data on sheet1 and creates charts based on that.

code snippet:

Set xl = CreateObject("Excel.Sheet")
xl.Application.Workbooks.Add
xl.Application.Workbooks("Object").Close
xl.Application.Workbooks(xl.Application.Workbooks.Count).Activate
xl.Application.ActiveWindow.WindowState = xlMaximized
xl.Application.Worksheets(1).Name = TableName

these kind of code is there in many forms and modules....
basically an xl object is created, reports are generated with the data from access.

Also, there are more than 100-200 excel sheets opened in the background (xl.application.visible = false)
Only when all calculations are complete, we make the excels visible (xl.application.visible = true)

Now, this works fine with excel 2003 and downwards but on a machine with excel2007, once you install the application and run, the calculations freeze and the application also freezes. It works fine when i run it on debug mode, one calculation at a time...

Any ideas??? I have a deadline in 2 weeks..