Start by creating 2 subdirectories - one for the VC5 DLLS and one for VC6. I created mine under the WINNT\SYSTEM32 subdirectory and I named them VC5-MFC and VC6-MFC.
Next, copy the DEBUG dlls for each version to the appropriate subdirectory. If you have
already installed VC6, it will have overwritten the VC5 versions, so you will need to copy
them from the VC5 CD. The files you need to copy (for each version) are:
xcopy C:\winnt\system32\vc5-mfc\*.* c:\winnt\system32
The USEVC6.BAT file would look like this:
xcopy C:\winnt\system32\vc6-mfc\*.* c:\winnt\system32
I created a shortcut on my desktop for each of these batch files so I could quickly run the appropriate batch file before launching a version of VC. I have been using this method for a couple of months (ever since VC6 shipped) and it works fine - no side effects or other strange behavior to report.
Procedure for applying service packs - When applying the VC 6 Service Pack 1, or any future service packs, be sure to run the USEVC6 batch file before applying the service pack. After the service pack has been applied, you need to copy the DEBUG dlls again from the WINNT\SYSTEM32 directory to the VC6-MFC directory.