If im not mistaken, you can install another OS it if you have an OS installed and even if the existing partition is the full capacity of your HD...
Try using Partion Magic...its a good program I used to use...Im sure you can create partitions even if you have an os installed/the partition is the partition is all the hd capacity becuase i used to do that....you only need to have some free space on your hd and youell it to create a partion..it uses that space and creates one....even if you already have partitions, it can take some free space from 1 partition and give to the other partition, or you can create another partition from the free space you have...
As for installing windows, i never tried that but there was an option in Partition Magic which sais so...i never tried it but it should work...
hope that helped...