Mouse acceleration was present but it seems to be fixed by adding (This is taken from The MarkC Windows MouseFix)

"SmoothMouseXCurve"=hex:\
    00,00,00,00,00,00,00,00,\
    15,6e,00,00,00,00,00,00,\
    00,40,01,00,00,00,00,00,\
    29,dc,03,00,00,00,00,00,\
    00,00,28,00,00,00,00,00
"SmoothMouseYCurve"=hex:\
    00,00,00,00,00,00,00,00,\
    fd,11,01,00,00,00,00,00,\
    00,24,04,00,00,00,00,00,\
    00,fc,12,00,00,00,00,00,\
    00,c0,bb,01,00,00,00,00

Under [Control Panel\\Mouse] 1588276333 in SteamApps/compatdata/906100/pfx/user.reg