Laptop cannot start(without system restore)

My 5 year old Dell Inspiron 1721, which had been running perfectly, turned itself off the other day while in sleep mode. Repair start-up didn't work so I used system restore, over 30 minutes later all is well. However, when I close down and restart I'm back to the same problem. I've restored to a number of past points but still no change.

I've also noticed that restarting the computer doesn't encounter this problem - it shuts down then restarts properly. Otherwise the laptop works great, buying a new one is no problem but I'd really like to be able to put this one right.