There are no stupid questions, so don't be afraid to ask. That's the way to learn! Image may be NSFW.
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Is this a GX520?
Did you get any warranty from overstock.com? If it's still covered, contact them ASAP. Some credit cards may extend warranties, so if o.co doesn't help, maybe your credit card company will pay for repairs?
First thing you might want to do is check the hardware inside. Power off, unplug and press/hold power button for ~15 sec. Then open the case and carefully reseat the power cable from power supply to hard drive, and both ends of the data cable at hard drive and motherboard. While you're there, carefully reseat the RAM modules and PCI cards in their slots too. Then close the case and reboot with only mouse, monitor and keyboard connected. Go directly into BIOS setup and make sure the SATA port is turned on so BIOS can see the hard drive.
Since you live in an old bldg, it might be prudent to get a surge protector, if you don't already have one.
Is the hard drive in the HP a SATA drive and what size? You would have to install that drive in the Dell, reformat it and do a clean install of Windows because the hardware is different and the Windows probably won't be able to boot the Dell without reinstalling. Everything on the HP hard drive will be lost by reinstalling Windows so your data must be backed up on external media first.
If the Dell came with XP, it's not legal to install Win 7 on it, unless you bought a separate license for Win 7 and haven't used the Win 7 product key on another PC. Dell doesn't support Win 7 on the GX520, so you'd have to do some hunting for Win 7 hardware drivers. That may be doable, but without knowing exactly what hardware is installed (eg video card, network card, audio card, etc) I can't promise you'll find Win 7 drivers.
Do you have a geeky friend who can help with the hands-on? Or do you know a reputable PC shop in the area, if you can't do this yourself? I can't see your PC from here. Image may be NSFW.
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Step-by-step Windows reinstallation guide
