Try to uninstall, clean all reg keys (if any) and install again. This should work.
By the way you may also want to download the latest version. http://www.gog.com/downloader

Also make sure you are in 'downloader mode' for downloading games

(see pic)

I never had any luck with the downloader, it always bugged out on me... I just download through the browser myself.. =/ Shame really, I wish the downloader did work for me because I would use it for sure.

Thanks for the replies all, I already done (if it's not three times) every single thing that everyone told me to do and nothing work. First time using gog.com and I don't think I'll use it in the futur.

I have the latest gog downloader version. I have tried restarting PC and reinstalling.

When I try to launch to download game or double click the GOG.com Downloader.exe;

Absolutely nothing happens. It was working just fine a few days ago.

What's up?

Okay, so.. read somewhere about getting .Net 3.5 so I did that but I think it's stupid because I'm on XP SP3 and it will auto update back to the latest..

I think what may have happened was in browser was on account/beta rather than just account.. weird but i think reverting back to just gog/account in browser url worked. Downloader seemed to work after that.

Maybe is a problem from GOG side, try to send an email.