So, approximately two weeks ago I noticed that some games make explorer.exe freeze. The first one where I noticed this was FireStarter and I thought it's a problem specifically related to this game. Then I noticed that it's the same for several other games, both GOG and non-GOG titles, I'm pretty sure that it also happens in case of games where I never had any issues of this kind in the past. I'm not running into this in case of newer game, it seems. At least Stalker: Clear Sky doesn't make explorer.exe freeze.

I did two Google searches, there's obviously tons of stuff about explorer.exe freezing but I couldn't find anything that really fits the problem I have. Any ideas what may be causing this? The only major change on my system I recall is installing CCleaner but I doubt it's related to that. I'm running Windows 7 64 bit with a GeForce 560 Ti here.

Hump. Argh, I meant bump!

GOG sometimes shuts down explorer.exe with a launcher to circumvent a graphics palette issue and starts it again when you close such a game. Since it happens with Non-GOG games as well, I doubt that's connected to your issue. Though it might be worth a try to deactivate any security suite, because it might not like such a functionality and interfere with this process.

Or maybe GeForce Experience is the baddie once again. If you're using this tool, shut it down or even better uninstall it to see if it makes a difference.
CCleaner has got a registry cleaning feature which potentially could damage something, but you'll probably never find out, unless you had backed up your registry before cleaning, thus being able to restore it to its former state now.

Curious... Recently when i was playing Starcraft i changed some settings for the shortcut, forcing it to 256 colors and 640x480 and the game looks and plays fine... So why explorer needs to die or stall...

Some old games using DirectDraw set their own colour palette which is consequently overwritten by Explorer.exe in newer Windows versions (or their corresponding compatibility modes) up till Windows 8, thus resulting in the famous rainbow colour effect. That's why GOG is shipping some games with a custom launcher, that shuts down the Explorer during gameplay and restarts it afterwards.