1) You will need to make Revit the default program to open this file. Please right click on the rvt file. In the new pop window, choose Revit and check the box for "always use this program". Please see image.
2/3) We have seen this issue when customers are using 32bit operating system. Very often this issue is resolved by installing a 64bit OS.
If your file is less than 20x bigger than the file you are trying to open, you will see that error. For example a 50MB file will need a 1000MB of RAM to open.
Others to try/check:
Check if you don't have any other programs stealing resources (like Pandora, many IE windows, etc..). Go to the Windows Task Manager and look at the Revit.exe process. Probably that is why it works when you restart the computer.
Clean your temp folder. Start>search window type %temp%>delete all files possible.
Check the minimum requirements for Revit. For 32bit OS, Revit 2014 needs 5GB free disk space and Revit 2013 needs 4GB of RAM
Try to enable 3gb switch.
Adjust the Virtual Memory to a max of 10gb.
Check if all updates are installed.
Hope that helps
Fernanda Lima Firman Technical Support Specialist Customer Service & Support Autodesk, Inc.