If you have ever used the XP hibernation feature, that creates a rather large file called hiberfil.sys in the root folder of your C drive. The size of the hiberfil.sys file depends on how much RAM is installed in your computer.
Start by just booting into the XP Recovery Console and just deleting the hiberfil.sys file.
Deleting that file is sometimes just enough to get your going again so you can reboot and then start cleaning up.
Note that when you boot into RC, you are limited to navigating only Windows folders - you can't get to any of your personal folders to delete things..