I was able to install Windows XP Professional on ASUS H81M-PLUS after the following changes in BIOS:
Advanced Mode - Advanced SATA Configuration - SATA Mode Selection:
AHCI --> IDE
Advanced Mode - Boot - Setting Mode - Secure Boot - OS Type:
Windows UEFI --> Other OS
(without producing special modified installation disk, what would be violation of EULA)
However:
1) Standby button doesn't work (is shaded out). Control Panel .. Display Options ... Power schemes don't work correctly too - monitor doesn't turn off automatically after requested time, and hard drive seems to not shutting down.
2) There is no video driver for Intel HD Graphics 4200 for Windows XP probably. XP works on its VGA driver, but it's icon is yellow in Device Manager (something is wrong). But colors look well (i.e. in YouTube).
And Google Earth can run only under Open GL, but not DirectX. Version 6.2.2.6613, because 7 and further don't cooperate with Windows XP, at least in some computer configurations.
Perhaps 1 is caused by 2. (?)
3) Some drivers must be downloaded from the internet and installed separately later
- LAN (Realtek® 8111G) and audio (Realtek® ALC887) drivers from the Realtek website
- USB 3.0 (Asmedia 104x) driver for two ports at the back