Boot Camp in Lion only have the drivers for Windows 7.
You need to get the drivers from SnowLeopard or Leopard.
Snow Leopard reference:
Step 4: Install the Boot Camp Drivers for Windows After installing Windows, install Mac-specific drivers and other software for
Windows using your Mac OS X installation disc.
The Mac OS X disc installs drivers to support Mac components, including AirPort, built-in iSight camera, the Apple Remote, the trackpad on a portable Mac, and the function keys on an Apple keyboard.
The Mac OS X disc also installs the Boot Camp control panel for Windows and the Apple Boot Camp system tray item.
To install the Boot Camp drivers:
1 Eject the Windows installation disc.
2 Insert the Mac OS X disc.
If the installer doesnt start automatically, browse the disc using Windows Explorer and double-click the setup.exe file in the Boot Camp directory.
3 Follow the onscreen instructions.
If a message appears that says the software youre installing has not passed Windows
Logo testing, click Continue Anyway.
Windows that appear only briefly during the installation dont require your input.
If nothing appears to be happening, there may be a hidden window that you must respond to. Check the taskbar and look behind open windows.
Important: Do not click the Cancel button in any of the installer dialogs.
4 After your computer restarts, follow the instructions in the Found New Hardware
Wizard to update your software drivers (Windows XP only).
5 Follow the instructions for any other wizards that appear.
6 Check for updated Boot Camp drivers by using Apple Software Update or going to
www.apple.com/support/bootcamp.
If You Have Problems Installing the Device Drivers
If it appears that the Boot Camp drivers werent successfully installed, try repairing them.
To repair Boot Camp drivers:
1 Start up your computer in Windows.
2 Insert your Mac OS X installation disc.
3 If the installer doesnt start automatically, browse the disc using Windows Explorer and double-click the setup.exe file in the Boot Camp directory.
4 Click Repair and follow the onscreen instructions.
If a message appears that says the software youre installing has not passed Windows
Logo testing, click Continue Anyway.
If you need to reinstall specific drivers, you can install one driver at a time. For example, if your built-in iSight camera isnt working, you can reinstall just the iSight driver.
Individual drivers are in the Drivers folder on the Mac OS X installation disc.
To reinstall a specific driver:
1 Insert your Mac OS X installation disc.
2 Quit AutoRun if it opens.
3 Using Windows Explorer, locate the driver that you want to reinstall.
4 Open the driver to start the installation.
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Is it the 64bit version of XP ?
If you are aiming for the 32bit version you will onły be using 4GB of RAM.
Sometimes it might be better to use a virtual machine instead of Boot Camp.
VirtualBox.org is free.