It means either
- your hard drive is getting full up.
- you have changed your page file setting recently.
- a large or buggy program is running that is eating all the resources (spyware ?)
Fixes -
- Right click My Computer/Properties/Advanced/Peformance/SettingsAdvanced/Change
and make sure the "System manged size" is checked.
- make more room on your hard drive.Uninstall programs,run CCleaner.
- Look in the Task Manager running processes list to see what`s running,
when you get the warning.