When i try to add King's Bounty to my Gog Downloader it gives error message "(EI41): bad signature"... all the other games works fine with downloader.

Also trying to download via browser has some problems. First it downloads all three part ok, but trying to install it gives error message about corrupt file.

Using Vista and UAC has been turned off.

Could you post the size of these three files?

three files:
setup_kings_bounty 2,23 mt
setup_kings_bounty -1.bin 1,95 Gt
setup_kings:bounty -2.bin 1,14 Gt

You have all three files in the same place, right?
The sizes are the same as mine and they install correctly.
I assume that the first file is .exe file. Try redownloading only this one. You may also try and skip the integrity check.

They all in the same place and i have tried to download all three files three times already, always the same message when i try to use this .exe file.

I wish GOG would provide checksums with their installers, it would make things so much easier for people to know if they've actually downloaded it correctly. Anyway, I would suggest this as step one, file size is not a sure way to check integrity. I am of course assuming that GOG give the same installer to everyone, and they're not doing some funky dynamic stuff depending on OS.

I used FastSum, but whatever tool you choose, it would probably help if you checked the MD5 checksum of your downloads. In theory the new downloader should do this, but it can't hurt.

097A819FB2974B855A44D7E76D2C3F19 *setup_kings_bounty.exe
DC2D07951C9AF3D199D8227EC014329C *setup_kings_bounty-1.bin
2A361A7CEFD012F7E05B18CF770DC80B *setup_kings_bounty-2.bin

Normally I'd suggest running the setup as an administrator, but with UAC disabled this shouldn't be an issue.
I see if I can dig up my Vista notebook but in the meantime you may consider opening support ticket, or maybe someone else will chime in with better advice.

Make sure it's not your Anti Virus getting in the way of the download and/or install.

I'm having the same problem. If I can't dowload this game I would like my money back!

The last email I got from support said that this problem appears to be something they have trouble replicating at GOG, and to try to download the game from another machine. I have other games I'm playing at the moment so I haven't gone through the annoyance of trying to download KB on another PC.

However, this was a very lackluster answer so far as I'm concerned. I would think they could at least try removing the game from my account and then readding it to see if that clears the issue, and in fact I will probably ask them to do that when I get back around to it. If the "use another PC" workaround doesn't work/isn't feasible for you, you might try to pursue that line with support.

EI41: bad signature (while trying to add to GOGdownloader

This seems to be a common problem.
any solutions or simple (!) workarounds?

I also had this problem on two different machines. However using different browser solved problem for me and gog downloader started correctly.

I've downloaded KB yesterday via the downloader without any problems.
(I run Win XP, if that matters)

This worked for me, too. Tried adding it to Gog Downloader from IE but got "(EI41): Bad Signature" but Firefox added it to the downloader, no problem.