Normally it shouldn't be a problem to start the desired Windows installation and remove the second line from its 'boot.ini' file. The second line of available OS's (the line that does NOT end with '/fastdetect'), I mean. When you see, you'll know.
After a reboot you can just format the partition that holds the undesired Windows installation (when that partition contains no data that you want to keep, of course). Else, you should be able to delete everything but the data you want to retain.