Problems with second Monitor on Windows 8.1

So I got excited and installed 8.1 on my work PC. Big mistake.

I use a Notebook (which works fine under 8.1) but because it it is 11 inch i use a External Monitor from HP. Apparently they haven't Updated their Drivers. I don't have a real clue what to do. I am pretty new to PC and the whole WIndows 8 experience after using a Mac for Years (which i still use, PC is my second Machine at Work).

I desperately need the Monitor for the PC, does anybody have a clue what could help?

Many Thanks in advance!