Dell Inspiron nearlyalways takes two attempts to start

I am attempting to make serviceable a Dell Inspiron 1525 laptop for a local charity. It was purchased at a car boot sale (for US forum members, it's a bit like a large public garage sale) and I have no idea of its history. I have restored the factory image from the recovery partition and, once booted up, the laptop seems to work well. Some of the front lights never come on and the touchpad buttons do not work so it isn't perfect but these problems are not important in use. However, the laptop almost always takes two attempts before it will boot up.

The first attempt very quickly gets past the Dell splash screen and then there is a horizontal progress bar and the words "microsoft corporation". The progress bar never stops but, having done a forced shutdown and restart, I get a screen offering me the option of a repair or a normal startup. The repair option takes a long time but ends by saying that a startup repair could not be done. Selecting a normal startup usually achieves this and subsequent startups soon afterwards also work properly. After an hour or more, the problem returns. This is my first experience of Vista as I am (said very quietly) an Apple man, so any suggestions as to what I can do to identify or fix the cause of this problem would be appreciated.