Have you tried singing it a lullaby?
Go to System Restore and have a look back through the Restore Points and see if there was anything installed a couple of months ago that may have changed the Sleep setting. I don't have access to Vista at this point in time but you can drill down into the power settings and micro manage them.
Another thing to try is at the Command Prompt (run as Administrator), run
sfc /scannow and see if that fixes anything that may have been changed. That won't change the settings but it will fix any corrupted or missing files that are associated with Sleep mode. [NOTE that there is a space between the
c and the
/forward slash].
In Windows 7, you go into (it should be similar to Vista):
Control Panel
Power Options
Change when computer goes to sleep
Change advanced power settings
[there is a Sleep setting and under that a Hybrid sleep setting. These may need to be changed. Make sure you write down or take screen shot of the settings so you know what they were before changing. ALT + PRTSCN will capture the active windows so you don't get the whole of the Desktop in the image. The Active windows is the one that has the dark blue Title Bar]
See if this helps.