... but with an old XP OS....
Virtualized XP or natively installed?
It could really be an issue with your keyboard. That's why I'm asking for virtualization.
Do you have any keyboard tools running, or any other tools (e.g. for graphics) which might catch Ctrl+Tab as a hotkey combination, before it gets to the game?
Also try disabling the Compatibility mode and check if it makes a difference.
It is indeed a natively installed XP, and no keyboard tools running that I know of, tried it with W95 / W98 and no compatability modes too, all pretty much with the same results.
However, I finially did get it do "cheats enabled" just the once!!!!!..... Then the "Can't Build More Units" bug came within a minute and I had to quit/reset the game anyway - not had "cheats enabled" back since!!!!! :O
It's looking like the game is completely unplayable due to these "Mission Failed" and "Cant Build Units" bugs however - I think I'm going to have to dump this one!
Ah well - thanks for being so patient with a crotchety old codger like me!!!!! Looking like this game ain't worth the hassle...... come back "Blitzkrieg 2" all is forgiven!!!!!!!!!!