Paging file size
Windows contains a feature designed to deal with performance issues when you're running out of RAM. The pagefile.sys stores memory for applications you have open, but have not used recently, so that more memory is available for the applications you are using. When you back to the applications after a long time, it should take longer for them to function again.
If this is really causing a problem for you while you're processing a lot of data, there is a way to adjust the pagefile.sys setting. Right click on Computer in Windows Explorer and choose Properties . . . then Advanced System Settings.
. . . next click on the Advanced tab, and then Settings in the Performance section. A Performance Options dialog box will open. Go the Advanced tab again and click on Change in the Virtual Memory section.
You'll know see how you can either deactivate the paging file size, or specify its parameters.
The drawback to doing this is that if you then do exhaust the system's RAM with the open applications, they will crash very quickly, since Windows cannot re-assign virtual RAM.