Well you need another partition on your hard drive, or add another hard drive.
WindowsXP uses a filesystem called ntfs, you may be able to resize the existing partition you have by using a program called partition magic, but it is a little risky, you could lose everything.
I'd highly recommend the adding a hard drive method, however one very cool thing about linux is it will be able to mount and read the windows hard drive, so files can be shared, with windowsXP's filesystem I think its read only though.Doing it this way means your windows will be separate and just fine.
At the end of the gentoo install you will install either lilo or grub which are little programs that will give you a choice at boot up as to which system you want to run.If it gives you a choice its probably safer NOT to install either one of these to the master boot record of your windows drive.Put it on a floppy or anywhere else it offers, usually there are at least 3 options there.
You will need to know about your hardware, video card specs, monitor specs, sound card specs, and type of network card if you have one.More than likely linux wont detect and setup all these things on its own.Dont worry, it happens in windows too, thats why your often issued a windows driver disk with some things for your system.You may have to goto the vid card site and get drivers for linux, sound card and NIC drivers will probably be included, just not installed by default.
I highly recommend visiting some linux sies and reading some tutorials, linux isnt windows, its basically a mini version of unix for a PC.It turns your system into a multiuser mini mainframe computer.All the unix commands work.Quite a bit can done from the command line, remember DOS, same deal, just way more commands and options.Learn it, you will need it, there's so much power there.Everything in linux is modular, you will have like 6 choices for browsers, several choices of window managers(desktops), and 2 versions of X, which is the server that makes graphics on your screen.
Dont be afraid, you wont hurt your system, keep an open mind and dont be afraid to learn, its not complicated but it is very different.Once learned though you will knowquite a bit about what makes a PC work, you will even understand windows better.
