I screwed up my pc?

all the ansrwer u got so far are right but first before anything reboot and put back in your drivers,then RESTORE your system

Well no NTLDR is no natural director... Beyond that I want to know where you found a 40 or 80gb internal drive. Anyway, when your pc boots, hit the keyboard button in the first 30 seconds after you push the power button that the screen says "=Boot Menu" or "=CMOS" or "=Bios," go to boot order, and tell the computer that the primary boot device "=Boot from CD" put in the new drive, and the operating system disk, start the computer and install the OS. But if your PC only had a 20gb hard drive, and I'm guessing its a desktop, you may just want to get a new pc. They are as little as a couple or three hundred dollars and have at LEAST 80gb of hard drive space installed, not to mention they are going to be faster. Good luck and hope this helps

Sorry if this sounds a bit vague, but with the info you have given it's the best answer I can come up with. It sounds to me as if your jumper settings on your hard drive are wrong. To reboot with the 20gb insure that NO other drives except CD are plugged in, and check the jumper on the 20gb to be sure it is set to master with slave. (you can usually find these settings on the top of the drive, if not you can find them on the manufactures web site.) This should work provided you haven't somehow reformated the 20gb drive.

sound like you replaced the c drive with another hd, you cant do that , you could put it in as a second drive (slave ), sounds like you will have to reinstall operating system