Troubleshooting inability for system to sleep
Here's a tip on how to troubleshoot the inability of a Windows 7 system from being able to enter sleep mode. If you install Windows 7 on a system and then discover that there's no option for entering sleep mode, check to see if Aero is enabled. If you can't enable Aero on the system, then you probably have the default SVGA video driver installed instead of the vendor-specific driver for your graphics processor. The inbox SVGA does not support sleep mode.
To resolve the issue, go to Windows Update and check to see if a vendor-specific driver can be downloaded for your graphics processor. If not, check your system vendor's website.