I own a hp pavilion slimline s3100n computer.

If your computer is less than TWO YEARS OLD, HP will replace the motherboard for free. See this web page: http://h10025.www1.hp.com/ewfrf/wc/document?lc=en&dlc=en&cc=us&docname=c01514044

Hi I'm actually working on a Slimline Pavilion at the minute and have checked out what u mean by the yellow light - indicates that the Hard Drive is functioning.

When the yellow light does not come on it means that it's not funtioning..... this may be because a cable has come loose or that the Hard Drive has been damaged.

You will have to check which it is, if there is a cable loose, put it back in securely. If all is securely attached and it still doesn't boot then there is probably a physical problem with the hard drive, which means you'll have to replace it and re-install the operating system.

Regards

I had a similar problem that when I turn on the power, the blue power light comes on but nothing occurs, even the bios page. For a while it succeeded to boot every several tries but it became necessary to try tens of times before it booted correctly. After it started the machine ran quite well, except when I make the machine sleep, then the machine would never wake up.
I found that the MB for s3100n has a common problem of this sort and HP even has sent out an email warning possible failure. When it actually happened to my machine it was too late: the warranty has expired more than a year. The replacement MB was above $250 and even if I buy it it is likely to fail again.
What I did instead was to get a microATX case with power supply, CPU cooler and a mother board and reuse all other parts from the failed s3100n. I wanted to reuse the DDR2-800 memory (my memory was upgraded) and athlon X2 4000+, so Asrock N68-S motherboard was a good choice. You can recycle HDD, DVD burner (SATA), card reader and wireless (internal USB), keyboard and mouse (PS2) as well. It was the first time I replaced a mother board but the case and mobo manual contained enough information to do the wiring. If your HDD is good Windows vista will boot in the new hardware. Get new drivers from Asrock/NVidia and install them to get the graphics and audio working correctly. Don't forget to re-activate Windows Vista using the product key which is stickered on the side of the s3100n chassis before you throw it away.

Forgot to say - the internal USB stuffs will not properly fit to microATX board and you would need some craftsmanship to make it look good...

I suggest you to try the steps in the below links:
http://h10025.www1.hp.com/ewfrf/wc/documentSubCategory?rule=21418&lc=en&dlc=en&cc=us&product=3440586
http://h10025.www1.hp.com/ewfrf/wc/documentSubCategory?rule=23280&lc=en&dlc=en&cc=us&product=3440586