|HP/Compaq supply a recovery partition which can be accessed by repeatedly pressing F10 at startup.|
this will bring you too the recovery options, repair/restore/factory recovery (if all else fails)
1st try repair,
2nd try factory restore
3rd if all else fails run a factory restore this will delete your C; drive and all your files but will set your machine to the day you bought it..
i hate computers!
but cant help myself....