I have a Dell Vostro 200 Slimline that I want to put another hard drive in. I have a harddrive out of a Dell Dimension E521 that I want to put in their, however, I don't want to lose my data on it. Currently the BIOS is set to RAID, but I only have 1 HD in it, what will happen if I install a 2nd HD?

I'm pretty sure that the E521 had IDE connections for Hard drives so the SATA connection on the Vostro 200 won't work; the drive probably won't fit in the slim case anyway. The RAID would only function if they two drives were installed with the OS at the same time so it is not a factor. You may try getting an external HDD enclosure suitable for an IDE to USB and connect the drive via your USB. It would be a bit slower but the old HDD is probably slower anyway. A better solution might be to get a USB2.0 to SATA/IDE cable adaptor and a newer external hard drive. Move the files on the old drive to the new drive and use the new SATA drive as a backup. Old hard drives don't ever last as long as you would like them to.
Good luck Tarael

The Dimension E521 I have has SATA, and it says SATA on the HD's on both PC's, so I don't think that is a problem. So if I just plug the new drive in it shouldn't bother any data even though RAID might be turned on in the BIOS?

What are the alternate options to the RAID option? I'm suspecting that if you have a look around your BIOS you will find that there will be other settings elsewhere that are far the purpose of configuring RAID. If these are not configured then I don't think yaw will have anything to worry about by just adding the other drive.

I couldn't find a manual anywhere for your motherboard, do you have one on a disk that you could upload somewhere?