I installed Mirc 6.3, but had issues with some scripts I had that would not work, so I uninstalled it, and went back to 6.2 version. But I have an issue where the registration splash screen from the 6.3 version still shows up when Mirc 6.2 starts, which stops the older mirc from auto-starting on computer startup. How can I get rid of that registration splash screen???

Re-do the uninstallation (I know it's not the 6.3 version that you'll be uninstalling).
Then go to the mIRC directory (for most versions of Windows it's c:\Program Files\mIRC)
Ensure that directory is EMPTY
If you're not sure, you can delete the directory, which will guarantee there's nothing left in the directory.
The re-run the installation for 6.2

NOTE: You will lose any and all settings, so they'll have to be reset, or you'll have to make a backup of the mirc.ini file and replace the mirc.ini file that's created with the back-up that you made.