how can i increase the size of virtual memory?

First of all you should have mentioned that which operating system you are using? Anyway you can increase virtual memory. but for this you have to set page size for operating system manually. if you can not set page file manually then reply me back I'll help you to set page file

The maximum amount of RAM you can install depends on the processor and the operating system. This page may help you in deciding how much you can usefully install: http://www.microsoft.com/whdc/system/platform/server/pae/paemem.mspx It is possible, though, that your PC cannot use more than 1GB You will also need to know the clock speed with which your PC accesses RAM, the RAM type and the physical size (and number) of the memory slots. Some older memory types MUST be installed in matched pairs. The quickest way to find out all this is to take the cover off your PC and to look at the memory installed. If you have two 128MB modules, then it's likely that it's a type that must be installed in pairs. Provided that your maximum RAM is 1GB, therefore, you should buy two 512MB matched modules of the same speed and type as the ones you already have.