Hello 3gm community

Since I've reinstalled Windows 7 I've had this annoying problem with my C partition filling up with I don't know what...

When I finished installing the OS, the C drive had 6  Gb free memory but since then it has been filling up continuously and now all I have is 1.5 Gb of free space and I'm already seeing the red bar alert. I tried CCleaner, Auslogics defrag, O&O Defrag, deleted restore points, ran windows Cleaner, fixed registry, scanned for Malware with Malwarebytes and viruses with Bitdefender... Nothing works, it still fills up, slowly.

I haven't installed anything on the drive except for Quake Live , since it's the only place it can be installed , but that only takes about 1 Gb or less if you count out the demos. I suspect it's from the Windows folder but there are dozens of folders in there , biggest being winsxs which is about 6 gb , but from what i know that's just an xp backup folder so win7 can run programs in different compatibilities right? How can i find out what folder it's coming from?

Please help as I'm running out of options and might go back to xp... thx in advance