Hello Connie,
Which version of LabVIEW are you using and which version of MS Office do you have? You should not have to register any ActiveX controls as long as you have Excel installed. If you are getting this error in Open Excel.vi, I would suggest you try recreating the block diagram. Drop an Automation Open function from All Functions -> Communication -> ActiveX and create a control for the Automation Refnum input. Right click on this control terminal and choose Select ActiveX Class -> Excel._Application. You can rebuild the rest of the code using Property Nodes and Invoke Nodes. Also this Knowledge Base entry might be helpful:
http://digital.ni.com/public.nsf/websearch/246B651C89F4AA2586256B1400656AE2?OpenDocumentFor writing a table containing ASCII characters to an Excel file without using ActiveX, you can use Array to Spreadsheet String.vi and Write Characters to File.vi. I'm attaching a simple example that illustrates this.
Also, if you have the add-on Report Generation Toolkit, you might want to look at the shipping example Sample Report (Excel).vi that illustrates how to write tables to Excel.
Hope this helps and let me know if you have more questions regarding this.
Good luck and best regards,
Shakhina P.
Applications Engineer
National Instruments