That could mean your HD's toast. Does it make weird sounds when you turn it on? If it doesn't, then some of your boot files are damaged, which can be fixed with the Windows CD (which you don't have atm).
My buddy's computer said that and he had to get a new hard drive and reinstall Windows. But his was making clicking sounds. This means he lost all his documents, pictures, etc.
But if this had happened before and went away by itself... then it could be a bad motherboard.

sounds like a corrupt HDD.

Try unplugging the HDD and plugging it back in (could be a lose cable). If that doesn't work I would suggest taking the HDD to a friends house and running a diagnostics on it (or if you have a linux boot CD which you probably don't).

Odds are you don't know how to do anything I said other than unplugging it. If that doesn't work, spend the $30 to buy a new small one, and install everything. It'll save you more time and frustration rather than trying to diagnose the problem.

Oh, and if you do get lucky and it boots, back everything up right away. You should have done that after the first time it failed.

lol, ok.

To back things up you either need DVD's or an external HDD. Then just either burn the files you want to save, or copy them to the HDD. You can get a "backup tool" but that isn't needed as you'll probably end up spending the same amount of time total either way. The only reason I'd suggest a backup tool is if you have either a ton of data that can be compressed (in other words not video, audio, or images) or if you back up regularly.

I'm assuming you have a standard tower PC.

As for the PC, first you want to turn off the PC. Generally there is a screw on the back and you slide the back panel off. Try not to touch anything on the inside, as I'm sure you don't have a ESD sensitive work area. Also, touch the case first before you start working and don't shuffle your feet around, or wear wool as you could fry the whole thing via static shock. As a FYI, you don't even have to feel anything to actually emit static shock and destroy electronics.

Anyways, Unplug the power cable from the PC and press the power button as we don't want to destroy anything and this should discharge the system. Then find the HDD, which should look like this.

http://www.newegg.com/Product/Product.aspx?Item=N82E16822148413

anyways, unplug all the cables at the HDD and plug them back in (there are 2, 1 power, 1 coms), and then do the same at the other end of the coms cables which should connect to the mother board. This should be easy enough to distinguish from the power cable which heads back to the power brick at the top of the tower.

Close everything back up, plug in the power and hit the power button.

If that doesn't work, I'd replace the HDD, as it is probably corrupt. You may be able to recover the data from it though if only the boot sectors are corrupt.

aw nordlead, you're really helpful ^_^ wish id voted for u in that competition thing now..

you can vote twice which is like a half vote for me

:p omg i was about to express my regrets for not voting for him either.+1 spirit vote for nordlead from me

i just didnt vote for u cos i didnt think you liked me..

but due to recent developments, (this thread), i see you are a good member not that my opinion really matters either way, lol.

Nooooooo!

Not teh Blue Screen of Death!