The pinyin in Windows 8.1 is much better than the one in XP, Vista and 7, finally the first usable MS pinyin after Windows 2000. However, it is too sluggish in Firefox and IE and it always crashes them on my Miix. I have no choice but, as always, go to Sogou 4.3 which is the only Chinese/English input method I can live with for many years now.
After Sogou 4.3 installation, you must replace two files, ImeUtil.exe and PinyinUp.exe, with dummy ones, otherwise it'll upgrade automatically to the new versions that suck. You also must disable the SogouIMEMgr in Task Scheduler.
If you use MS OfficeXP on Windows 7/8, Sogou 4.3 is not compatible to that combination. You ether upgrade your Office XP or, like what I do, use Windows 8.1's build-in pinyin just for OfficeXP. Switching between these two IMEs are very convenient with Win+Space.