Excel (VBA) UserForm & Macros from Office 2003 to Office 2007 causes errors. Any ideas why?

I have an Excel program which I designed in Office 2003. This program worked perfectly for a few years in Office 2003, but I am now trying to run it with Excel 2007 and have run into some problems.

It has a UserForm object named "UserForm1". In this UserForm, I have a Frame which I named Spreadsheet1.

Every time I try to run the program, the debugger throws the following error in UserForm1's code:

"Run-time error '438': Object doesn't support this property or method"

Me.Spreadsheet1.Cells.Clear

I also get the same error when I just try to run the following simple command from a different Module:

UserForm1.Show

ANY IDEAS would be GREATLY appreciated! I am completely stumped! :(