If this drive was a boot drive in another system that had a different chipset then what it is in now you will have to try a repair install. This will reinstall Windows but all your programs and personal files will remain unharmed.
 
Normally when you put a drive in a system with different chipset (VIA to Nvidia, SIS to Via, etc.) it is best to do a fresh install of the operating system.
 
Reason is the hardware is different.
 
Repair Install of XP
 
 

And, given that you do successfully repair the installation, you may have to reactivate or re-register your copy of XP since a number of of your components have changed and XP thinks you have a pirated copy.

o.k. I did try to start it up with my XP CD in the CD Rom and the HD attached to this system, but it does not go into the start up mode of the CD. It goes right into trying to boot up, then I get the blue screen flash, then it goes right into restart mode again. I did try to switch the start up sequence to boot from the CD, but unless I am doing it diiferent, it also isn't doing it. How would make it so I can get to the start up mode of the CD from this point?

You will need to go into your BIOS. When you reboot the computer or whenever it starts up, you will see a message like hit DEL or F9 to enter the BIOS. When it comes up, you will see one of the options is Boot sequence. Change if fro C:\ to the CD rom.

O.K., I tried changing the boot sequence set up under the bios, but for some reason, the CD will not read. My friend has offered me to borrow his XP PRO CD to do it, but I am not sure if this might produce a problem with what I want to do. I mean, if I borrow his, can I still use my CD key if it asks, or will I need his? And if I do use it, will it conflict with my HD seeing as I had initially installed the OS with my CD?? OH MY GOD, PLEASE HELP ME! AAAAAAAAAAAAAAAAAHHHHHHHHHHH!!!!!!
 
HAHA! I just don't want to lose any of my stuff. Ecspecially when it is also a perfectly good 40gb HD. I mean, this shouldn't be this hard, should it?

At this point you don't know what shape your hard disk is in anyway since it crashes whenever it gets to a certain point. All you actually want to do is get to the Recovery console to get XP to accept the new hardware it finds and reconfigure the drivers for your new machine within the parameters of Windows when it boots up. So, try to see if you can get his CD to work. If it doesn't work, you may have a faulty CD drive. If it does work and allows you to proceed, it will either let you "repair" or not, and it will probably tell you to call Microsoft to re-register.
 
Your second question was "it shouldn't be this hard?" The answer is that XP is configured with the hardware it finds on a machine when it is first installed. This scheme was to insure that the program wouldn't be pirated or used on more than one computer. So, yes, Microsoft made it this hard.