Create & Use a UDF Without a Workbook Reference

I am going berserk trying to do something that I would expect to be straightforward. I have searched for the last 3 days to get directions, but have run into numerous false leads and dead-ends.

I have a UDF (call it MyUDF) that I would like to use in any workbook I open on my computer (only) and I'd like to refer to it via only the function name without a qualifying workbook name (e.g. =MyUDF(args) instead of ='MyUDFWorkbook'!MyUDF(args)). I have had this working in the past (used in 50+ workbooks), though I must admit to not understanding/remembering how enough to be able to recreate it. I had programmed the UDF in a uniquely named workbook I saved as an Excel Add-In (i.e. with a .xlam extension). I'm not wedded to that approach, but am willing to use it again. I broke this several days ago when trying to overcome an Interop.Excel exception I was getting in a C# Winforms app I'm creating.

I have read about and tried programming the UDF in PERSONAL.XLSB instead of a uniquely named workbook, overtly putting that workbook or my uniquely named one in the Excel AddIns and/or XLSTART folders (under my Windows user name), setting the IsAddIn property somehow (I could find how to do that), using the Add-Ins manager (which is still showing the name of my UDF even though I've deleted it from both the AddIns and XLSTART folders), and other things. There are moments where things I've tried seem to work, but then they don't when I shut down Excel and reopen the workbooks that use the UDF.

I'd really appreciate guidance on how to clear out any residual junk I may have created and start over to get this to work. I'm using MS Office Home & Business 2010 on a Windows 8.1 machine. Thanks.

Thanks for the attempt to help, but I'm afraid I don't understand or something else is going haywire.
1) I have my UDF in an xlam in a personal folder. The xlam file has the same name as the UDF (surely that's OK).
2) I renamed the xlam file to hide it from Excel. I went to File / Options / Add-Ins / Manage Excel Add-Ins Go. I unchecked the box next to MyUDF which triggered a message saying it couldn't find it and asking if I wanted to remove it. I said yes and it went away.
3) I closed Excel. I then reopened Excel and went back to File / Options / Add-Ins / Manage Excel Add-Ins Go. I Browsed to and selected my xlam file with the new name. MyUDF reappeared in the pop-up list of available Add-Ins with the box checked. (It's the only one checked of the 5 Add-Ins that are shown). I clicked OK.
4) I closed Excel and opened one of the workbooks that uses MyUDF. Excel asked if I wanted to Update the links to other sources. I clicked Update. It said I had one or more links that couldn't be updated. I clicked Continue. I looked at one of the references to MyUDF. It was fully qualified with the name of the original xlam file. I edited the formula by deleting the file qualification. It still showed a #Name? error. I tried to fully qualify the UDF with the path to my xlam file. Excel accepted the new formula, dropped the path info and left only the name of the xlam file as the qualification. This new formula still had a #Name? error.

Once again, I'm at a loss. As a 1960s mainframe systems programmer who was away from technology for 30 years and taught himself C#/.Net/etc in the last few years, you would think I could figure this out. The evidence suggests otherwise. [P.S. I'm thinking about buying help to get around this problem, but can't find out what it means to buy 20 points for 30 days for $20. Grrrr.]

Again, thanks for your quick and thorough help. My module name is different than my function name, so no problem there.

I appreciate your suggestion for getting past the qualified function references. With more than 50 workbooks containing references, that's a bit of a pain, but then this whole thing has been a pain. Since last looking at your response, I have discovered that I can open each workbook, click Edit Links when prompted, select the UDF workbook, click Change Source, browse to and select my xlam file, click Update Values, and save the workbook. That too eliminates the qualified function references. Fingers crossed this problem doesn't come back again.

There seems to be no doubt that Excel or some other part of Windows is storing some value that keeps it from automatically picking up the most recent version each time. Who knows where that might be or how to overcome it (if that's even possible).