Can someone please tell me if it is likely to be possible to overwrite Activated Vista Ultimate Pre-SP1 with a new OEM Home Premium I have just bought (this may seem strange to wish to do this, but I do). I do not want to have to delete the Drive holding existing version completely before installing the new one unless it is absolutely necessary, it also contains all my software. Thanks.
I suspect that you have Home Premium which includes SP1? Otherwise there would be no point in downgrading.
I am not sure if it is possible to downgrade a version of Vista but all you have to do is insert the disk and see if you get that option. If it won't then all you have to do is either update your Ultimate to SP1 or wait until Windows Update will allow you to do so.