This tip might also be applicable to Vista but I'm not sure.
In the task manager on the "processes" tab, you can see a list of running processes, how much CPU they are using and how much memory they are using. If you click "View" and then "Select Columns", you will see additional columns of information that can be displayed on the "processes" tab. I HIGHLY suggest showing the "Virtual Memory Size" column since that will provide more memory information than the standard "Memory Usage" column that is displayed.
Today, I worked on a Windows 2000 machine where Windows was complaining the system was low on virtual memory and a program I was wanting to run couldn't since it couldn't get enough memory allocated. I went to the "processes" tab in task manager and could account for about 75MB (high water mark) of RAM that was being used even though a little over 1GB was _actually_ allocated. Once I enabled the "Virtual Memory Size" column, I could easily identify the process hogging 750MB of memory (virtual memory) and terminate it to restore the system to a normal operating state.
If this is old news for you, sorry for wasting your time.
If this is new news for you, send me a PM where I can send my bill.
Peace...