Setup issues the error, "Another program or the operating system is locking a file Setup must update."  I can select Yes, No, or Cancel.
-or-
Setup issues the error "An access violation occurred while copying the file."  I can Abort, Retry, or Ignore the error.
Note:  The Addit! Pro 3.1+ Setup program should be able to update most locked files.

Applies to:  Mfc42.dll, Msvcrt.dll, Msvcrt40.dll, or other file.
Explanation:  Setup is trying to update the file but can't overwrite it because the operating system or another program is locking it.
Resolution:  Close the program(s) locking the file. Press CONTROL-ALT-DEL to bring up the Windows task list. Use End Task to remove unneeded programs, especially applications like Cleansweep and virus scanners. The two to keep are Explorer and Systray. Now re-run the Setup program. If the problem persists, the operating system itself could be locking the file (as is likely the case with MSVCRT.DLL if you're running Windows 98 or NT 4.0+).

If after closing all programs and tasks Setup still has trouble updating the file, download the Core Components Update (http://www.byteforge.com/products/ap_fs98/support/article_ap98s7.htm):

The Core Components Update (V1.01.0002, last updated June 16, 1999) will install or update the following files:

Filename New Version
MFC40.DLL 4.1.6140
MFC42.DLL 6.00.8447.0
MSVCRT.DLL 6.00.8397.0
MSVCRT40.DLL 4.22.0000

Installation

1.  Download COREUPD.ZIP (ftp://ftp.byteforge.com/ap_fs98/coreupd.zip) (1.8 MB).
2.  Create a temporary folder on your hard drive.
3.  Unzip the contents of the zip file to the temporary folder.
4.  Run Setup.exe and follow the instructions. 

Setup may ask you to restart your system. If so, select 'Yes'.

After installing the Core Components Update, re-run the Addit! Pro Setup program.

If all else fails, ignore the error. Setup will tell you the application may not function properly. The only way to find out is to try it.