Finding the starting place is difficult... Every tech shop has a slightly different approach.
With that model, I would suspect the hard drive first, based on our experience in repairing them... but that is the most costly if you are wrong unless you have a nice stock of hard drives.
Without the recovery disk set, it can cost you more... unless you have a version of windows handy... You can install without registering so as to test the drive...
It is not a secret that keeping good notes of what you try, and what happens when you make the attempt, can help you in narrowing down the possibilities... Sometimes you can get so far into the diagnostics that you forget what helped and which didn't, then you may find you need to refer back to an earlier diagnostic...
Sometimes components that are not even installed can be part of the difficulty, such as a defective modem or sound card.
Were it in our shop, we would test hard drive, then one memory module at a time.
Do not discount dust, dirt, and fiber, cpu fans, case fans, modems, video cards.
Start as simply as possible... e.g. First look for dust, dirt, and fiber, cpu cooler fan, then setup as one hard drive, power supply, and one memory module.
When you buy a hard drive to test, you will always be able to use it later on... as all hard drives fail.
This hard drive list shows relative reliability in our shop (could be widely different elsewhere) beginning with most reliable: Seagate, Western Digital, Fujitsu (laptops), Toshiba (laptops), Samsung, Maxtor, Hitachi, Tri-Gem. We would automatically replace any Maxtor, Hitachi, or TriGem without even testing,
Of course, you will learn a great number of other approaches as the day runs on.
