Though I always advice clean installs to anyone installing linux but if u don't want to load win again, wht u can do is, as advised by cheyenne1212, use partition magic to create a separate partition for u(be sure to defragement ur drive first) adn then load linux on it. thoggh id still advice for a scratch installation. If u r interested, i'm ready to give u exact details. It'd definitely save u a lot of troubles later on.

Bludgeon Said :choose your C: slice as the installation's destination

U can't do that as in that case, ur C: drive will be formatted to ext2 and win xp boot loader will be removed. U have to create a separate partition(or install on some partition other than C) for ur linux to save ur xp boot loader. BTW, in any case i always recommend creting a separate smallish partition for the boot loader instead of installing it on the MBR