[Roy Dennis]

I believe You should be able upgrade any of the MTU programs if you bought the original within 12 months so check the date of your original install from your account.

Are you trying to install from the upgrade page which is here: http://www.mtu.com/upgrades/microstudio.htm

Then press the upgrade button for 4.24, when the Save or run option shows select Save when it completes select Run and enter your registration info you used for 4.23, it will check all the information you enter and either install or inform you that your upgrade time has expired and how much an upgrade is.

If you choose not to upgrade you should still be able to load your existing version providing you saved the original file.
Do you not have the original 4.23 installation file ?
You are advised to choose the save option when you first download the program, not the run option. So you should have chosen a folder to save the "mstudio423setup.exe" file to on your computer so if you download your programs to a certain folder look in there for it. If you can find that file on your other computer then copy it to your new computer and start it from there entering all your previous registration details you had before and it should register OK.