You have a Dynamic Disk which will not support XP until it is converted to Basic. This is a destructive process which requires moving the data off of the drive then converting to Basic using this process: Change a dynamic disk back to a basic disk: Storage Services; Local File Systems
Before doing anything else, you should make your Acer Recovery disks first so you have a path back to factory condition.
If you wish to install XP and don't want to move your data off and convert the disk to install XP and reinstall Windows 7 (in that order) then I would either run XP virtually within Windows 7 or attach a second HD to install XP.
If you choose to install XP to a second HD, I would unplug the Windows 7 HD during install, then when XP is installed boot via the BIOS boot order or one-time BIOS boot menu key. This keeps the HD's independent to come and go as you please. A Windows-managed Dual Boot configured by EasyBCD 2.0 interlocks the HD's and makes it harder to remove one.
For example, after installing XP I would plug back in the Windows 7 HD, set it as first HD to boot in BIOS (after optical drive), then if I want to boot XP I would tap the F2 (on Acer) key repeatedly at bootup to bring up the BIOS Boot Menu to choose the XP HD to boot.