How to reformat?

The easiest way is My computer > Right click the drive you wanna format > Format > (optional) Quick Format > Start.

That'll only work on drives that don't contain the main operating system. That's not really reformatting.

When you say that you want to reformat a drive, I'm assuming that you want to install a brand new operating system and want to clear the drive of all the data. If that's the case, remember to back up all your important files/folders.

Reformatting is actually quite easy. All you need to do is to stick in the Installation CD and restart your computer. Your computer should then boot from the disk and you'll be given instructions on how to reformat from that. If you're using Windows XP, it should be really easy. Just select the partition that your OS is installed on, and press 'D' to delete and 'L' to confirm. Then when you're formatting the new partition, make sure everything is NTFS. That's pretty much it.

I can get you a guide, but I'd like to know why you're reformatting. Are you going to be re-installing your current operating system so it'll work a bit smoother?