explorer.exe uses full core pemanently, ntdll.dll!RtlValidateHeap+0x17

As soon as I open Windows Explorer, it uses a full core of my 4 core system. I'm running Windows 7 Pro 64 Bit. Process Hacker shows ntdll.dll!RtlValidateHeap+0x170 as start address for the thread that uses the processor resources. I've tried Process Monitor to find out what this thread is doing, but only the thread exit with success (after I've terminated it in Process Hacker) shows up in Process Monitor. A System Restore has brought no change. I can't find anything suspicious in Event Viewer.

How can I find out what causes this explorer.exe behaviour and resolve it?

I've run a full scan with both Microsoft Security Essentials and Anti-Malware. If these two cannot find the culprit, what else can I do?

I posted a reply here on Sunday, but somehow it seems to have been lost...
I've tried most of the software at http://www.bleepingcomputer.com/download/windows/security to no avail.
When I boot in safe mode with pretty much everything disabled, Windows Explorer works fine. After having wasted several days with useless scanner software I've installed an SSD drive and installed Windows 7 on it, which works fine. The disk on which the infected Windows 7 is installed is now used purely for data storage, although I can still boot into the infected Windows if need be.