In Windows Vista

1. Click Start button Picture of the Start button
2. Click Control Panel
3. Choose System and Maintenance and then click System.
4. In the left pane, click Advanced system settings.
5.On the Advanced tab, under Performance, click Settings.
6. Click the Advanced tab, and then, under Virtual memory, choose Change.
7. Click Custom to change the Initial size (MB) and Maximum size. See the hot tip in windows xp


Remember to choose the drive you would like to use to increase the virtual memory. In most cases it usually your C: Drive. You can set the initial size and maximum size after clicking the Custom size check box.Remember its better to keep the Virtual memory values the same.

Also having enough available space is absolutely necessary for your virtual memory and RAM to function properly.