Windows 7 has hibernation turned on by default. Hibernation creates a hibernation file on your hard drive with the same size of your RAM. When hibernating Windows will store the memory contents in this file. If you do not plan on using hibernation you can turn it off manually:

1.Click Start, and then type cmd in the Start Search box.
2.In the search results list, right-click Command Prompt, and then click Run as Administrator.
3.When you are prompted by User Account Control, click Continue.
4.At the command prompt, type powercfg.exe /hibernate off, and then press ENTER.
5.Type exit and then press ENTER to close the Command Prompt window.

More information and a Microsoft tool to turn hibernation on or off can be found on the link below:
http://support.microsoft.com/kb/920730