Running Vista, I have installed the Canon provided RAW codec for CR2 format.  It allows you view those files natively. 

I was browsing through a folder of pictures.  I noticed that it got slower and slower as I went through the pages.  Eventually, it gave me an out of memory error.  I didn't have anything else open.  I brought up Task Manager and saw that I had memory left.  I closed the picture viewer and just watched the memory get reclaimed in about 20 seconds.  The total delta there is about 1.5 gig.