New Advanced Format hard drives are being made with 4096 byte sectors, rather than the current long-time standard of 512 bytes. While this won't be a problem for Vista, 7, MacOS or Linux, this will prove to be a problem with Windows XP (and older Windowses). XP doesn't support anything other than 512 byte sectors, so it can't use these new drives. To get around this, the drive manufacturers have created a system where the drive pretends to have 512 bytes while still maintaining 4096 bytes internally. While NTFS normally works in 4096 byte clusters, the partition clusters don't quite line up with the sectors on the drive, forcing the drive to read each sector, modify the data, then rewrite the whole sector. And it has to do this twice for each cluster. This makes data writes several times slower than a current generation hard drive. There is software that can realign the partitions, but even then there will still be a substantial (~10%) performance hit.
So if you're planning on installing a new hard drive in the near future, you may need to upgrade your OS.