when i open up several programs, a message is been displayed on a screen,
that is "UR computer is low at virtual memory, increase memory"
what is meant by that You don't have enough memory (RAM) and the size of your paging file is too small.

The following quote is extracted from this site.

Quote

How to manually change the size of the virtual memory paging file
You must be logged on as an administrator or as a member of the Administrators group to complete this procedure. If your computer is connected to a network, network policy SETTINGS MAY also prevent you from completing this procedure.
1.Click Start, click Run, and then type sysdm.cpl in the Open box.
2.Click the Advanced tab, and then click Settings under Performance.
3.Click the Advanced tab, and then click Change under Virtual memory.
4.Under Drive [Volume Label], click the drive that CONTAINS the paging file that you want to change.
5.Under Paging file size for selected drive, click Custom size, type a new paging file size in megabytes (MB) in the Initial size (MB) or Maximum size (MB) box, and then click Set.