My suggestion would be not trying to setup the Linux partitions with Partition Magic; I would resize the Windows partition, then run the Linux install; I believe all modern distros have enough of a partition manager to create what it needs, even if it can't resize what's there. But you'll need enough unpartitioned free space on your hard drive to do that.
Also, most modern Windows XP machines (well, many at least) have NTFS partitions; while it's better than the FAT32 alternative (what Windows 98 & family used), it's not as well supported; few distros are going to have built in support for resizing NTFS partitions without wiping everything out (actually, that's a big deal of Mandriva Linux - it's DiskDrake partitioning program can use NTFS).
Also, if you get into a problem with booting, you'll want a bootable partition manager disk. Partition Magic might have one, I've never dealt with that program personally. My suggestion would be a program called BootIt NG, from Terabyte Unlimited (http://www.terabyteunlimited.com/bootitng.html
); technically it's shareware, but it never expires and it lets you move, resize, and copy your partitions; it also will rebuild the MBR for you.