Thank you. This led me to the right results. For those who have this problem in the future, I'll sum it all up in this post.
Originally, I started out with a Vista machine. Recently the motherboard died which meant a major hardware change for my computer. Once the motherboards were swapped, I was stuck. Vista doesn't have any real repair options which meant either a reformat was necessary, or I needed to get a new drive and just make the other one secondary. I chose the latter.
This time I decided to take the XP root. The installation went smoothly, but then I noticed every time I rebooted the machine it would always ask for the CD. If there was no CD in the drive, it would give me an "INSERT SYSTEM DISK" message and would fail to boot.
When I rebooted with the CD in the drive and accessed My Computer in Windows, I noticed that the larger partition (the new hard drive) was marked as E.
Now, the fix (and thanks again, Eric!). While Windows was installed on E, the partition still wasn't marked as active. So I went into disk management, right-clicked on the partition, selected "Mark as Active", rebooted, and problem solved.
One issue I didn't mention was that the new drive, although 300GB, only came up as having 137GB. Upon installing service pack 2, the machine detected the rest as unallocated space (which was discovered while marking the XP partition as active). From there, I was able to partition the remaining space and fully utilize my hard drive.
Thanks again!!! :-)