SOOOLVEEED!!!!
So, I've been thinking. What has changed in the system between the time GIMP was running fine and the time it didn't? Well, long story short, I remembered that I got a new mouse. I know, this is probably something I should have mentioned it in the first place since it is a significant change to the working environment after all. But honestly - we are talking about normal USB mouse for $5,60, one that you just plug in and it works - just like the one I've been using before. Thus, I didn't give it too much thought, and immediately forgot that I had new equipment installed.
So, to get to the point, I uninstalled the drivers, both from the old one I haven't been using (
Here is an interesting guide on how to do it properly, if anyone in the future is looking at this thread) and the new one (it installed them immediately again, since the device was connected).
And well, right after that I tried to open GIMP... and it DID open this time.
Eh. See, there was absolutely no other visible problem caused by these drivers, and the mouse itself has worked perfectly. This is the exact kind of stuff which made me go into power engineering and not CS.
I'd really like to thank you all for your time and wisdom,
Despite the fact that the cause turned out to be something different from what we expected, I learned a lot in the process. I will never forget this place. ^^