How do you swtich hard drives?

Make an image of the drive to a second drive or partition using Aconis True Image (not free) or similar like Macrium Reflect Free Edition Drive Image.
Then restore to the new drive.
WD & Seagate supply a free version of Acronis for the job, I don't know if it works on other makes or if Samsung has one of their own.

Ok, thanks. This just got A LOT more complicated. His old piece of crap computer only has one cord coming from the power supply that can feed a hard drive. I guess I could put both hard drives in my new build and just transfer all the files over (can i do that?). Like just click and drag all of the files to the new drive.

Don't make this as complicated as Jonmor68 wants to. All you should have to copy is the users profile, under Documents and Settings in XP, or under Users in Vista. Install YCopy, (freeware), on the new drive and create a folder to copy to. Hook up both drives, fire up YCopy and in the "FROM", point at the OLD drive/users profile, and in the "TO", point at the folder created on the new drive and start the copy process. Once done move the old data to the new users profile; IE, Docs to Docs, Desktop to Desktop, Favorites to Favorites... It's really pretty simple.

Alright, So if theres 4 users on his computer do I need to do that process 4 times? Once im done moving all the stuff, I dont need that old hard drive anymore right?

BEFORE you do anything drastic to the old HDD, make very sure the machine works with the old drive removed from it. I am worried that the "simple" process described has not copied everything yet. So check before destroying your ONLY backup on the old drive!

Before you do anything, does his old computer support the new drive? Is it IDE or SATA?

Older systems, pre-2003, don't support 48-bit LBA, so hard drives are limited to 137GB. And Windows XP doesn't support hard drives over 128GB before SP1.

To get over the power thing, get a spitter. A molex or SATA Y-splitter is only a couple of bucks at your local Computer Store.

Cloning via acronis, is the smartest easiest way to do it.

And if you have so little knowledge of what to do, and dont know how to add another power adapter to the PSU, you really should stop now, because if something goes wrong, your gonna be with out a clue fast, and your friend will hold you accountable.