My htc one will cannot connect to pc when going into bootloader mode

I downloaded a ruu file for my htc one because my phone cannot be updated maybe because my dad rooted it long ago.I already unrooted it but still it cannot update.So my friend told me that i have to flash rom in it.I downloaded the RUU file,It can run but whenever it click the next,my phone will reboot.I dont know is it suppose to happen,but it will auto disconnect from my pc.But when it enters a black screen writing htc,it will connect again but my pc will say that device is not recognised.I've tried using other solution but most of them need some sdk and adb thing which i am very confused bout it.Any help would be appreciated.Thank you.