I have a 2.7 gig processor with 1 gig RAM.  The OS is Win XP Pro w/SP2.  I do a tremendous amount of audio editing with no problem but I have started using Adobe Premier for video editing for our ministry.  All programs are turned off in the background including anti-virus, spyware, etc.  No screen saver activated; nothing!  My memory will go to absolutely zero.  I use Tune-Up Utilities 2006 and do keep the memory optimizer open.  The gauge will bottom out; go below the red line.  I have completed all the diagnostics on the machine; no virus detected, no adware or spyware detected.  I have uninstalled and reinstalled Premier several times but it makes no difference.  The verson is 6.5 if that makes a difference.  Any ideas of what could be draining the RAM?  I know that Premier is a huge program but this is the only program with which I have a problem.   I use Audition for the audio editing with no problem.  

Before beginning using the program, hit ctrl/alt/delete. This will bring up the task manager.
Click on the processes tab. This will give you a list of the processes running on your computer and show the amount of memory they are using and the amount of processor time they are taking.
For example, I have 47 processes currently running on my computer and only 1 program currently running. There is approximately 250 MB of memory being used. Many of these processes are programs waiting in the background.
You can shutdown many of these processes, if you have administrator rights.
I suspect that is where much of your memory is going. Shutting off a program does not automatically shutdown the processes associated with it.