When using the Windows XP installation disk, you can manually allocate partitions. If it's a really old computer, dedicate the majority of space for Linux (Mint LXDE is best on old hardware, but to a certain extent. If it's REALLY
ancient, you might want to try putting SliTaz on it; boots up in 35 seconds smack on a AMD Duron 900MHz, 168MB RAM).
If you want to install Mint first and Windows later, do as Vincent said:
1. On the Mint live cd/dvd, when you are installing and get to the 4th step, click on "Something else" or "Advanced" option.
2. Click on the harddrive (which is probably called /dev/sda#) and delete any extra partitions (
if any exist) so you have one huge partition.
3. Click "Add" and make a 1GB (1024MB) swap file (this is similar to Windows' pagefile.sys). Click on the dropdown menu and click "linux-swap".
4a. Now whatever space is remaining on the harddrive, dedicate half to Linux and half to Windows (if its a small harddrive, dedicate two-thirds of it to Linux

).
4b. Now, for the Linux partition, you can put any of these filesystems: ext2, ext3, ext4. Each latter filesystem is better than the preceding one from what I know, so pick ext4 (If you want to learn more about the similarities and differences, go to this page
Clicky.) From the dropdown menu, choose ext4 filesystem, and for the mount point choose "/"{slash} or "root" is what it means.
5. Now hit next and wait for installation to finish. Thats how you install Mint, and most all kinds of Linux OSes in general.
Now, onto the Windows XP installation:
1. When the 10 page license agreement comes, ignore it and hit (F8? I think) whatever it says to press to agree to the license agreement.
2. When the partitioning page comes, select the unallocated space, and press enter.
3. Format it as NTFS if you are worried about not being able to use or store files single larger than 4GB; otherwise, choose FAT32.
4. Wait 1 hour for it to install (go get and eat lunch), and restart, and you're done.
5. That's how you install Windows XP.
Big issue after installing Windows XP: the bootloader will not recognize Mint and give you a choice to boot into it.
1. Download and install EasyBCD
http://neosmart.net/EasyBCD/2. Open the program as an administrator. Click on "Bootloader setup".

3. Now click on the tab called "Linux/BSD". Click on the drop-down menu and select "GRUB2". For the name, type in Linux Mint. Now hit "Add entry".

4. After you click "Add entry", restart your computer and you will be presented with an option of booting into Windows XP or Mint.
5. That concludes the Windows/Mint dual-boot tutorial.
Enjoy Linux!