You can install VB5 and VB6 on the same PC.  However, when VB6 upgrades the
ocx controls, in  some instances it screws things up.  I have found this
problem with the comctl32.ocx (Windows common controls) and the comdlg32.ocx
(commond dialog control), and it may occur with others.

You create a project on a PC that has only VB5.  You check it into
SourceSafe and check it out on a PC that has VB5 and VB6 installed.  You
open it in VB5 and it works fine.  You save it and check it back in.  When
you check it out and open it on the PC with only VB5, you get error messages
that it cannot find comctl32.ocx and comdlg32.ocx.  The messages show the
path to the folder that the project is in, not the system directory
(c:\myproject\comctl32.ocx insted of c:windows\system\comctl32.ocx).  If you
attempt to set references to the controls, you run into problems compiling
the program.

I have found the following workaround:  After you check out the files on the
VB5-only PC, open the  .vbp file in a text editor and delete the references
to comctl32.ocx and comdlg32.ocx.  Then open the project in VB and add the
components back.  Save the changes to the vbp file and close the project.
Then when you open the project again it will work fine.