Vista - Update Problems

My Vista Home Premium, keeps shutting down.
When I put it into sleep mode and then reactivate it shuts off.
Then it comes back on in safe mode, have checked all in Device Manage, but they are all ok.
It also goes off sometimes when not being used and you start using it again.
Has a screen come up with start up repair recommended which i did, this has happened a few times.
The diagnostic message was all was successful and said that 'unspecified changes to system configuration, might have caused the problem'.
At the moment my pc is working, but it was like that yesterday for a long time. Any ideas? I'm a complete newbie so please keep it simple. E. Clarke.