Considering the fact that Windows XP is still the most-used OS worldwide, anyone considering swapping out their old hard drives for a new one should heed this advice: be careful. You may want to even consider not doing it.
The problem rests with the hard disk sectors, which are the smallest unit of a hard disk. Each sector, or block, has 512 bytes, but has a certain percentage of "wasted space," which can't store anything. The new 4K sectors that are being ushered in before January 2011 (the deadline the International Disk Drive Equipment and Materials Association has given manufacturers) have less than eight times the wasted space, with each sector given more than double the capacity—4,096 bytes, to be exact.
Using XP with new drives could slow down the write-time noticeably, but more advanced drives will trick XP into thinking it still has 512-byte sectors. If you do have a PC running XP, and want to upgrade your drive at some point, Ars Technica's write-up on the issue is a verbose, detailed look and definitely worth your time.
Failing that, just buy a new laptop running Windows Vista or 7, OS X Tiger, Leopard, Snow Leopard or Linux (after September 2009), as there are no 4K hard disk compatibility issues there. [BBC via Ars Technica]
Image Credit: Barjack