if it does'nt let you make a partition in the unpartitioned space then just use the already partitioned one that has windows on it. it will tell you, you can not have two windows on the same partition, but you can.
windows will normally be :C\windows , when you install windows again on the same partition, it should ask you to put it somewhere else. you can then put in :C\windows\ two << this can be what ever name you want
the second install of windows will now go to :C\windows\two
and after it's all installed, reboot the computer , right after your computer logo disappears keep hitting the ESC key to bring up the list of OS's, it should be like this:
Windows xp << this one will be the one installed to :c\windows two
Windows xp << this one will be the one installed to :c\windows
just highlight the one you want to boot-up & hit enter