Picasa has one big known bug which has not been resolved for the last year, that is it's viewer executable is not killed when you close it. So, if you want to use it to view pictures, you end up with tens of threads left open. Each will take 5MB RAM circa. It happens on both 32 and 64 bits versions of Windows since XP on, and the net is full of complaints not being addressed, so far.
Consider that my PC gets rebooted only when updates require me to, so basically I end up with tens of dead threads taking each 5 MB of ram...