You need to install from old to new for it to function properly...
So you'll have to install XP on whichever partition first than reinstall Windows 7 on the second partition.. The boot manager will configure itself and everything should be fine as long as you install them in the right order..
Also, a good app to tinker with the boot manager is EasyBCD (available here: Download EasyBCD 1.7.2 - NeoSmart Technologies ). You can set one OS as the default OS and shorten boot selection times, etc with this app..
It's not something you HAVE to have but it's definitely worth checking out.. Good luck..
Operating System Windows 7 Ultimate 64- Bit Edition
Computer Type Custom Build 6 of 6
CPU Type and Speed AMD Phenom II X4 965 (3.4GHZ) 4000MHz HT Quad-Core Socket AM3
Motherboard Chipset Asus Crosshair III Formula AMD 790FX Socket AM3
System Memory Type Corsair TR3X6G1600C8D Dominator - DDR3-1600:PC3 -12800 - 6GB - Non-ECC Unbuffered 240-Pin DIMM
Video Card Type and Speed Sapphire Radeon HD 5870 1GB PCI-Express (2.0)
Computer Monitor Samsung SyncMaster P2350 23" FULL 1080p HD LCD Monitor
Hard Drive (2) WD VelociRaptor 300GB 10000RPM 16MB Buffer: (2) WD Caviar 1.5TB "IntelliPower" 64MB Buffer
Optical Drives (2) LG 22x Super Multi Drives (SATA II) - Both with Lightscribe
Network Adapter Asus Crosshair III Formula Gigabit Ethernet Adapter
Anti-virus Software AVG Free Edition 9.0
Computer Skill Level Self-Taught Expert
Windows Experience Index 7.2