The thing you call a command prompt is the power-on self-test (POST). Because the POST is screwed up and not just the OS, it's probably your motherboard that is screwed up. When buying a new motherboard you need to find one that fits your CPU socket, RAM, case, and various expansion cards. You might not be able to rebuy the same one you have now, they may not sell them anymore (this is pretty common).
The other thing it could be is the hard drive. The way to test this is try putting your hard drive in another computer. If it works then the drive is fine and it's definitely the motherboard. If it doesn't work then it means the drive is bad, try replacing the drive in your computer first before getting a new motherboard. It's possible that both the drive and the motherboard are bad.
Online stores will have a bigger selection of motherboards, if you want to try a brick and mortar store then you'll need to go to the nearest Fry's Electronics/CompUSA/Micro Center or your neighborhood computer store. You won't find motherboards at Circuit City or Best Buy or some place like that. RadioShack occasionally has some computer parts (RAM, fans, various accessories, and specialized parts are common) but I don't think you'll find a motherboard there. Your best bet is online.
If it's still covered under a warranty complain to Dell or the store, whichever place the warranty is from.
