HP laptop continually restarts

[SunflashX]

I bought my HP Pavillion dv9000 laptop about 9 months ago, a refurbished model from Buy.com, and up until now, it has worked fine. Today, I left my computer idling while I was working on my desktop, and when I went back to it, the screen was black and I couldn't do anything. I held the power button for a few seconds to let it restart, but it restarted to a black screen, with some of the lights on the keyboard coming on and what I think is the hard drive going, but then immediately shut down again. Not enough time to even enter safe mode or anything like that. After a few seconds, it started up again, only to do the same thing. This would've gone on forever, so I took the battery out, and tried it on AC power. The same thing happened when I powered the computer on again, so I was forced to disconnect it from AC power as well. I've also tried disconnecting the power and battery and holding the power button down for 30 seconds, but this didn't help. I've also tried reseating the memory to no avail, though I might try this a couple more times just to make sure. Help?

Specs:
HP Pavillion dv9000
3 gigs of RAM
250 gig hard drive
AMD Turion 2.00 ghz dual core (something like that)
Integrated Nvidia 7100M graphics card
Vista Home Premium

[Broni]

Let it cool down for a while, and see, if it'll start.

[SunflashX]

The laptop is pretty cool now, but it still won't properly start up. I've also now tried using each RAM stick separately and in each slot, none of which has worked, so I don't think it's a RAM problem.

[Broni]

Did you actually try to start it in Safe Mode?

[SunflashX]

I repeatedly hit F8 once I powered it on, and it didn't work. No text ever appears on the screen before it reboots itself, just black.

[Broni]

Do you have any warranty on it?

[SunflashX]

No, the 90 day warranty has long run out unfortunately. At this point, I'm thinking it might be a problem with the power supply or something else serious like that. How much would it cost to have hardware inside a laptop repaired/replaced?

[Broni]

It's impossible to say, since we have no clue what's wrong.

If you have more than one RAM module installed, try starting computer with one RAM stick at a time.
NOTE Keep in mind, the manual check listed above is always superior to the software check, listed below.
If you have only one RAM stick installed...
...run memtest: http://www.techspot.com/vb/topic62524.html

Run hard drive diagnostics: http://www.tacktech.com/display.cfm?ttid=287
Make sure, you select tool, which is appropriate for the brand of your hard drive.
Depending on the program, it'll create bootable floppy, or bootable CD.
If downloaded file is of .iso type, use ImgBurn: http://www.imgburn.com/ to burn .iso file to a CD (select "Write image file to disc" option), and make the CD bootable.

NOTE. If your hard drive is made by Toshiba, unfortunately, you're out of luck, because Toshiba doesn't provide any diagnostic tool.

[SunflashX]

The laptop is pretty cool now, but it still won't properly start up. I've also now tried using each RAM stick separately and in each slot, none of which has worked, so I don't think it's a RAM problem.

Tried starting with each module separately already. I can't run any diagnostics because I can't get past the blank screen.

[Broni]

Tried starting with each module separately already

Most likely, it's not RAM then. BTW, memtest creates bootable CD.

Run HD diagnostic, which will create bootable CD as well. This way, we'll see, if the CD will give you any screen display, and if so check your HD.