@Agama - Over 6 million calibre users, probably 85%+ on Windows, and I see a grand total of 4 people reporting having a problem. Yes a lot of people won't notice if they do have it, but there are enough regular users visiting these forums you can be darn sure will have checked to see whether it also happens to them when it was first reported. I have Windows 7 x64 too with no issues - and if neither I, Kovid or any of the other "calibre regulars" are seeing the issue, then I call it a minority
And yes it can be a needle in a haystack exercise - which is precisely why only people like yourself experiencing the problem can provide the input necessary to resolve it.
I'm not saying that there isn't a problem "somewhere" btw - but speaking as a software developer this sort of issue is almost impossible to resolve without *quality* input from the users. It could literally be almost anything as the cause - some misbehaving plugin, some feature of calibre that only a subset of users are using, some conflict with other software on your machine causing calibre code to go awry, issues within third party libraries that calibre uses and so on.