Computer can't hibernate

My Dell latitude d 600 can't hibernate. each time I try to put a check on the 'hibernate' check button, I get a message that looks like this ''THE PROCESS CANNOT RESERVE THE HIBERNATE FILE''. please can any one help me out on this issue. some times you don't need to shut down completely so you could continue from where you stopped, the next time you switch on but due to this issue, you would always start from the beginning. I'd really be glad if I get a response as regards this. thanks in anticipation