START->ALL PROGRAMS->ACCESSORIES->SYSTEM TOOLS->SYSTEM RESTORE
IN THAT YOU WILL SEE ATLEAST TWO OPTIONS ON RIGHT SIDE i.e
o Restore my computer to an earlier time
o create a restore point
select 1st option and click next
now, you will see a small calendar to left select that date in calendar when your keyboard was working fine .
NOTE : only that date would get selected which u will see in dark colour which matches the day when ur keyboard was fine
now click next and next again .
your pc will shutdown and restart automatically after this
Hope this will work