See many other threads. I would download the SP3 to ,say, desktop (don't run). Then disconnect from internet; disable firewall, anti virus and spyware,(probably easiest to Run, type msconfig, then start up, then disable all). then restart. then install SP3 from desktop. Then msconfig again, -re - enable start programmes, re enable anti virus etc. then restart, reconnect to internet check for MS uodates.