Windows Vista
If you receive warnings that the virtual machine's memory is running low, will have to increase the minimum size of the paging file. Windows sets the initial minimum size of the paging file on the amount of random access memory (RAM) installed on your computer plus 300 megabytes (MB) and the maximum size at 3 times the amount of RAM installed on your computer. If warnings appear these recommended levels, increase the maximum and minimum sizes.
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Open System by clicking the button Start
in Control Panel in System and Maintenance , and then click System .
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In the left pane, click Advanced System Settings . If you are prompted for an administrator password or confirmation, type the password or provide confirmation. 
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On the tab Advanced in Performance , click Settings .
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Click on the tab Advanced , and then click Virtual Memory , click Change .
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Clear the Automatically manage paging file size for all drives .
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In Drive [Volume Label] , click the drive that contains the paging file you want to change.
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Click Custom size , type a new size in megabytes in Table Initial size (MB) or Maximum size (MB) , click Set , and then click OK .
Increases in size usually do not require the computer to restart, but if you reduce the size, you must restart the computer for the changes to take effect. It is recommended not to disable or delete the paging file.