to turn it on:
right click on my computer,
select properties or "personalize" as it says in vista,
go to the advanced tab,
from there click the "settings" button in the performance box,
go to the advanced tab,
click on the "change" button in the virtual memory area,
select custom size, enter the starting and maximum size, (mine are both 2048mb)
MAKE SURE YOU CLICK "set"!!!!
A windows should pop up saying you need to restart before it changes, click ok.
Then Click "apply" from the advanced tab. restart.
Test these settings by running the same program that caused the message. if it doesn't show the message, you win!
If it does, Get windows 7.
EDIT: to see if it is enabled another way, make sure you can see protected operating system files, then go to the drive C:/ and look for pagefile.sys. If its there your all good. then turn off the showing of protected operating system files.