Yes - I've tried another keyboard on this computer (exactly the same type of keyboard) and still have the problem.
I've also tried the keyboard on another computer, also with Windows 7 on it, and that computer does NOT have the problem.
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This affects all the control keys - Ctrl, Alt as well as Shift !!!!
For instance if I hold Alt and Tab and then release the Tab key, the available windows to switch to are shown for about a second and then I go to the window I'd selected.
I've installed W7 on a 'server' (HP Proliant ML115) and there aren't any keyboard drivers listed for non-server OS's, so it may be a keyboard driver issue after all.
Are there any others out there with a similar setup?
RESOLUTION
It is the keyboard/keyboard driver. The keyboard is a PS2 type, the HP one that came with the ML115.
I'm now typing on a USB keyboard and do not have the problems any more !!!
Thanks for your suggestions, ones I should have tried before bothering you in the first place really, but maybe this will help someone else.