I am planning to build a computer for a friend. He wants four 400gb HDDs because he wants to turn his computer into a TIVO (basically). My question is: Will Windows XP Professional be able to make full use of the 400gb HDDs? (I have already found a compatible motherboard with enough ports, and their support team told me the motherboard would be able to handle the size.)

Large Vendors are selling XP Pro systems with both 400gb HDDs as well as 800 Raid 0 configs (2 400gb drives) so I would take that to mean yes.

I have done some more research and learned that having (and seeing) hard drives larger than 137gb is dependent up whether you are using 48-bit addressing. Both the motherboard's bios and operating system must support this. I know Windows XP home with service pack 1 or 2 as well as Pro with 1 or 2 support hard drives greater than 137gb. The maxiumum for these drives is over 100,000pb (petabytes, or over 1,000,000gb). Now I can have some peace of mind knowing it these hard drives won't cause me a problem.