Look under things to try here: http://support.microsoft.com/kb/978610
The first thing mentioned is:
Check the system configuration settings
This problem may occur because the Maximum memory option is selected incorrectly. To fix this, follow these steps:
1.Click Start Start button, type msconfig in the Search programs and files box, and then click msconfig in the Programs list.
2.In the System Configuration window, click Advanced options on the Boot tab.
3.Click to clear the Maximum memory check box, and then click OK.
4.Restart the computer.