Running both Sentinel for HP and Sentinel for new x36 series both under Windows 8.1 Enterprise x64 and no issues either UI or functional. I know it does not help you but just reporting for sake of reporting.
Windows 8/8.1 does not change much underlying dll or kernel components from Windows 7, at least none that would break somewhat benign programs like Sentinel.
So here is the usual set of checks:
--Was your Win 8/8.1 an upgrade from 7? If not, maybe some drivers for your particular hardware are not fully implemeted or well written. Check your Device Manager for errors.
--Do you have the latest Graphics drivers for your video card from the manufacturer? The one's from Windows Update can be a few versions behind
--Have you tried installing Sentinel in a NON standard location? That is, outside of C:\Program Files\ ?
--Have you tried disabling UAC?
--Does it work fine in Compat mode?
Get your apps and devices working in Windows 8.1 - Microsoft Windows Help
--Try creating a brand new user profile and then installing and running Sentinel as the new user and see if it runs any better