Which version of Microsoft Office? Which version of Win8.1, Home or Pro, 32-bit or 64-bit? The information asked for is very important to know.
You can't Upgrade to Win8.1 from WinXP, has to be a clean install which wipes out everything. You may have lost your documents but should be able to Install your Office program from the disc.
I've had success installing Office 2000, Office XP/2002 and Office 2003 [at least the basic/most used functions] and later versions on Win8/8.1.