OK, I'll try my hand at this one. A tool that its developer claims it's useful for this work is
HeapMemView. Give it a try. And I suppose you can use Process Explorer in combination with Process Monitor to pinpoint the possible culprit, using the performance graph of Process Explorer (it logs "Private Bytes" history, along with a timeline) and Process Monitor to trace the app(s) working at that point time (and what are they doing). Process Monitor also has "Boot Logging", but that one could be a nightmare to filter...
If it is a shell extension, it had to be leaking like mad. Well, I hope this can help, at least until f0dder comes along