Hi i had the same prob with someone's pc i fixed.
You have to fisrt make sure there is no viruses on the pc(i scanned with bitdefender and superantispyware, you can use trendmicro housecall too but that also tell you about all the missing updates) then when i found no more viruses i installed bitdefender(latest version and did another full system scan), then did a repair install of windows, And that sorted it for me.