All you need is some free unpartitioned space on your hard disk. If your hard disk is one big partition like most people's, then you have several choices. If it is formatted in FAT32, there is a free Linux program called parted that can non-destructively resize your partitions. It is probably on the Red Hat disk, if you boot in "rescue mode", you'll be sent to a command line, from which you can run parted.
If you your drive is formatted in NTFS, or you'd rather go a more user friendly route, you can use Partition Magic to resize the paritition. Alternatively, you could delete the existing partition and reinstall Windows on a smaller partition (which will result in the destruction of any data on the partition).
I'd recommend giving Linux at least 5 GB or so.