Windows virtual memory is where windows stores data from RAM when it requires more RAM for programs and RAM installed on your PC is insufficient. By default windows stores this data on c:\pagefile.sys. And windows manages this pagefile size. This may lead to slow computer or performance decrease if you don’t have sufficient RAM.

To overcome this set the pagefile manually.

Right Click on My Computer -> Properties -> Performance Options -> Virtual Memory -> Change

Click on Custom and set this to 1.5 times your RAM. You can also select drive where the pagefile is to be set. I suggest to put this on drive other than C:\ for increased performance. Make sure the drive you set the pagefile has enough space. Sometimes this limit of pagefile may be crossed due to heavy application and then Windows increases this pagefile. If it doesn’t find space then your PC will freeze or may show up errors. If you have more than one hard disk you can set this pagefile on other hard dis. This will result in increased performance.

You can also use PageDefrag to defragment pagefile. Read more about this here