Hard drive problems

I'm having problems with my computer. When I start up my Dell desktop PC, sometimes Windows doesn't boot up. I do get the BIOS messages, but then the computer is blocked on a black screen with just a blinking underscore cursor in the upper left corner. Turning the PC off and on again two or three times then finally results in Windows XP booting from the hard disk. Afterwards the computer works fine. A bit like an old car that has problems starting, only that the Dell is just over a year old.

As the error occurs before Windows even boots, and before data are being read from the hard drive, I'm pretty sure that the problem is not a virus or similar software failure. Something could be wrong with the BIOS, but I haven't modified that for a long time, and besides changing the boot order I wouldn't know what settings to change there. I fear that the problem is with the hard drive not starting for some reason, some hardware problem with it.

Another possibility would be that the error occurs because I changed my keyboard. The old keyboard went into the keyboard socket. The new keyboard goes into the USB socket. Maybe not connecting a keyboard to the keyboard socket causes some problems, I know that old PCs wouldn't start without a keyboard connected. But then you'd get a keyboard error message, and I don't have that.

Now I am a bit at a loss what to do. I only took a one-year guarantee for the Dell, which is just expired, no wonder the PC breaks down now. :) Anyway, I wouldn't want to have Dell repair it, because they would need to send UPS to get the machine, then it would be away for an unspecified time, and then I'd need to stay home again when UPS brings the machine back. And all that costs far too much money, especially since they are likely to just replace the hard drive. I could just as well buy a new hard drive and install it myself for half the money and a fraction of the time. Only if it then turns out that it isn't a hardware failure of the hard drive, but some other problem, I spent the money for nothing.

I already did one sensible thing, made a backup of my data to DVD. I also ran Checkdsk on the hard drive, but didn't find any errors. I tried to find people with similar problems on Google, but didn't find anything, because there is no error message, and the problem is difficult to describe. I could go to a local repair shop, but the error only occurs occasionally. Nothing worse than trying to show an error to a repair guy and the computer works perfectly at his place. Any idea what I could do to better diagnose the problem?