Try restarting the computer and as it boots up keep pressing the F8 button every second or two until you get into safe mode. Once in Safe Mode, go into the command prompt and type 'powercfg -h off' (without the quotes) at the prompt and then press enter.
If you can't do this I have read that it is to do with the hibernate button malfunctioning and apparently it is a well known problem.
Let us know how you get on.