My experience:
This is probably a hardware problem if it appears intermittent.
The simple things, presuming it is a desktop:
Open the case and make sure everything is seated properly.
Use the Microsoft memory tester although RAM problems usually cause BSODs.
Freezing startups are often due to impending power supply or hard drive
failure.
Hence if you can start your computer backup immediately any files you will
need, the entire hard drive if you can.
Most hard drive manufacturers have software on their website for testing the
hard drive. Often when a drive fails a software test, especially the newer
giant hard drives, it is because a sector is bad but it happens to be in a
frequently accessed area. This may be fixable with a reformat preceeded by a
surface scan that blocks out the bad sectors. If the drive is in warranty
and fails the software test the manufacturer will usually replace it.
Power supplies that come with cases often are just plain lousy, underpowered
and fail frequently, more so than hard drives. If you have another computer
switch power supplies. If that solves the problem buy a quality name brand
power supply no less than 350 watts but better in the 400-500 watt size. If
you have a Dell, or other machine that uses a proprietary connector to the
motherboard you may have to get a power supply through them; some third
party units come with adapters but that might be risking the motherboard.
