Haven't made much progress (no time for dealing with Calibre) but wanted to thank you for the link.
The more I read the more I tend to agree that the magnifyer (Zoomtext) + the graphics system are good suspects.
I am now really curious how this will work on a Windows 7 64bit system with the newest ZT version.
I also noticed that recently there seem to be a black out (as mentioned in the linked posts - no blue scren just a immediate shutdown) when using a backup programme and trying to download a bigger file from a certain website (and only this site). However I managed to do both with ZT switched off and suddenly there were no black outs at all.
I am not sure what exactly this means but it's ... interesting.
No idea what a graphic adapter actually is or how to change it but I know someone who knows quite a bit about computers and hopefully knows what to do with the mentioned possible causes of the problem.