This is one reason that Windows becomes more and more unresponsive the longer you keep it running, and why you end up eeding to restart your computer periodically.
Yeah, gotta restart *Windows* because *applications* leak memory. What about just restarting FF and IE? :-P
Once you determine which process is using more virtual memory than it should, you need to determine what the process is. Some are pretty obvious, like winword.exe is Microsoft Word, iexplore.exe is Internet Explorer, and so on. Others aren't so obvious. The best way to determine the source of a specific process is to use a search engine to search for the Image Name as shown in Task Manager.
Which usually gives way too many junk hits these days
. Protip: select "properties" for the process in task manager, and then "general" tab will show you the location of the process... often more useful than sifting through the junk search results.
Also: install sysinternals' Process Explorer instead of the cruddy old task manager, it's so much better