First make sure UniFi is not running
Locate your java installation directory.
On 64-bit, it’s usually at “C:\Program Files (x86)\Java\jre6\bin”; otherwise “C:\Program Files\Java\jre6\bin”. (replace jre6 for jre7 if you have the latest Java)
Add the dir above to the PATH (from Computer->Properties->Advanced system settings)
run a command prompt (as an Administrator, right click on ‘Command Prompt’ and choose ‘Run as administrator’. THIS IS IMPORTANT ON WIN2008/7+, otherwise the service may not get created)
java -jar lib\ace.jar installsvc
Start the service: net start “Unifi Controller”
I tested this today and it works with xp just fine.