When trying to start MySQL the CP shows...
ERROR: Status check failure 
This program must be run from your XAMPP root directory.
This has been well and truly covered before and this topic holds several remedies for the error message:viewtopic.php?f=16&t=44320
Also you could use the new version of the control panel found in this post:viewtopic.php?p=180145#p180145
If you copied over the \xampp\mysql\data folder in total and replaced the one in 1.7.4 then this may well be the cause of the error as you should only have copied over your own database folder(s) not the mysql installation database folders.
First try and move out of the \xampp\mysql\data folder the data folder(s) you copied over from 1.7.3 leaving only the default databases (folders) that were present after installing 1.7.4 and then try and start MySQL again - this testing should always be carried out after an XAMPP installation to test that the installation itself has no issues.
Then, if the installation has no issues you can try adding your own files and folders again and restarting MySQL after each addition to pinpoint which addition may be the culprit of the error message - you should only copy over your own databases not any of the 1.7.3 installation databases.
The default database folders in \xampp\mysql\data\ after a successful 1.7.4 installation should be as follows:cdcol
Your database folders will have the name you assigned to them when you created your databases.
You may have to reinstall 1.7.4 if you still have problems but save only your own database folders and any files and folder from the htdocs folder and then follow these instructions for an alternative method:viewtopic.php?p=179699#p179699
If you don't want to do a full installation again then you could download the ZIP version and extract only the database folders from the \xampp\mysql\data folder in the archive to the corresponding folder in your 1.7.4 installation and then add your own
database folders and try again to start MySQL.
You may still have to recreate users and passwords for your own databases if the copy over of your own database folders was successful.
Do you still have your own 1.7.3 files and folders backed up?
I bet you will in future.
Then no problems.