If you have gotten this error for Rainbow Six 3 multiple times and can't start the game again.
"Ran out of virtual memory. To prevent this condition, you must free up more space on your primary hard disk."

- Log off of Windows and Log Back In or Restart your computer.

If its still happening a lot, then give this patch a try only IF your windows is 64 bit
It allows the game to recognize 4GB of Virtual Memory instead of only 2GB on 64 bit systems.
http://ntcore.com/4gb_patch.php - Run it on your Ravenshield.exe file to patch it.