Trevor Law wants to partition his hard drive, but the manufacturer has warned him that doing so will make it impossible to reinstall Windows.
Unfortunately, the manufacturer is probably right. Many companies ship PCs with a hidden recovery partition on the hard drive. This partition contains an image backup of your C: partition as it was when the PC left the factory. Should Windows become hopelessly broken, that partition is the default way to restore the operating system.
Twice now I've seen repartitioning render the recovery partition inaccessible. It's still there, but you can't boot to it and therefore can't use it. For what it's worth, both were Dell computers that shipped with Windows XP.
I've written two blog posts about repartitioning recently, one for XP and one for Vista. I admit that I didn't discuss this problem in either. On the other hand, I did recommend making an image backup of C: before partitioning. This will give you a better image than the one that came with your computer, since it's a record of Windows as you like it, not the way your vender wants you to like it.
In fact, after you partition your hard drive and move your data to the second partition, make another image backup, just of the now-smaller Windows partition (probably C:). That will be the best recovery tool you could have.
One more suggestion: Contact the PC's manufacturer and see if they'll sell you a recovery disc (or set of discs) to replace the partition. Some do, and the newer your PC, the more likely they will do this.