I'm trying to open an xlsx document in Windows 7 Pro 32bit with Office 2003 SBE and the latest Office Compatibility Pack and it doesn't recognize the xlsx file.

I have found the problem/s:

The immediate problem: The version of Office 2003 didn't have Service Pack 3 installed. Once I installed it the xlsx opened fine.

The underlying problem: I also didn't have Microsoft Update installed and enabled. Windows Update doesn't grab Office updates so despite the Windows 7 OS being up to date the version of Office was as installed. So another tip is to make sure you enable Microsoft Update as it will pick up old versions of Microsoft Software not just the O/S as Windows Update does.