Hello tackyjan. I'm not the dev of XW but since I'm part of the XW team and I use the program for ~2years I think that I can tell you my opinion.
I read carefully all your post. First of all, all this time no one report installation issues, that's why seems really strange.
As you know microsoft give windows updates that very often causes compatibility problems with programs. Don't forget that win8.1 is a totally new platform so maybe it's a reason for your problem. In the other hand, I suggest you to take a look at the windows installer and the registry for corruption!
XW v2 is under development with new look, new features, so keep in touch..
Now about the rest, you are a new user and is logical that you have some difficulties to understand how the things work!
I suggest you to take a look at these sections:viewforum.php?f=10viewforum.php?f=7viewforum.php?f=8
where you can find tutorials and a lot of help/informations/assistance about the widgets and XW in general.
Do not confound the "theme" function and installation with the single "widget"!
A theme is a set of widgets that are saved as a single file.viewtopic.php?f=10&t=2315
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