Backup your BootCamp. Then use the BootCamp utility in OS X to erase the BootCamp partition. After that, partition and install Window again. The OS X partition will not be affected at all.
EOS 40D+BG-EN3/350D/17-40 f4L/70-200 f4L IS/50 f1.8/18-55 f5.6/MacBook&ThinkPad