Since the days of DOS, hard drives have always been comprised of blocks of data 512 bytes in size that are read in a chunk. But that's to change in early 2010, as storage manufacturers have agreed to shift to a format that instead applies 4096 byte chunks. However - the format is incompatible with Windows XP
The reason is that each cluster has to have a small gap between it and the next one, as well as an area dedicated to error correction. That means wasted space, and is why a hard drive that's supposed to be 500GB will often show up on your computer as several gigabytes short of that. A move to 4k chunks means that there's less wasted space, and more data can be given over to error correction. Hard drives should get between 7 and 11% bigger as a result.
But the 4k format wasn't decided until after Windows XP was released, meaning that the operating system doesn't support it. That could spell trouble, but the new drives will be able to fake the 512 byte blocks - meaning that the drives will still work - albeit rather slower than before. Windows 7, Vista, OS X Tiger, Leopard, Snow Leopard and versions of the Linux kernel released after September 2009 should all work fine.
If your hard disk is malfunctioning, however, perhaps it's possessed. According to a book published by a Georgia preacher, "Any PC built after 1985 has the storage capacity to house an evil spirit", and he estimates that 1 in 10 computers in America are infected in such a way. If you've got an issue that Geek Squad seem unable to fix, maybe you ought to try an exorcist instead...