Ok what happen in your case was that you updated to service pack 3 which also included a bug. Not a virus bug, but a Microsoft bug in their coding of one of the KB updates. What is happening is that the Microsoft Update Agent (wuauclt.exe), which is part of the svhost uses 99% cpu power. You can check it by hitting CTRL+SHIFT+ESC on the processes tab. There you will see that a svhost is using 99%. However, ignore the system idle process using 99% this is normal.
The first thing you need to do is stop windows updates by going to Start, Control Panel, Microsoft updates. Change to off and hit apply. Go back to processes and high-lite the svhost that is running at 99% and right click and end task. Next you need to check which version of IE you have. Open your IE browser and go to help and about. If you have Version 8 then you need a patch KB2879017. http://www.microsoft.com/en-us/download/details.aspx?id=40390 After Installation reboot the system and then go back to control panel and re-enable Microsoft updates. Verify if the fix has been resolved. Try going to Microsoft updates to see if it loads.
If you have Version 6-7 you need to download IE 8. Be sure to download the proper 32bit or 64bit. To check which OS you have press Hold down the Windows key & press Pause Key. The information there shows which you have. Now download the IE 8 you need Under English section http://windows.microsoft.com/en-us/internet-explorer/downloads/ie-8. Reboot your system after install & then re-enable Microsoft Updates in control panel. Verify that the fix has been resolved and go to Microsoft Updates and see if it loads. If not apply the KB patch above and try again. It's hard to say which exactly works and in which order. There isn't whole lot of input or direction from Microsoft on this issue. I have found it to work one way on some systems and others different. I had one that wouldn't patch at all and still had issues. Other things to try...
If there is still an issue try to Install Microsoft Updater 3.0 http://technet.microsoft.com/en-us/library/bb932139.aspx
Some other KB patch if nothing else works. KB927891 http://www.microsoft.com/en-us/download/details.aspx?id=6458
Microsoft is ending XP support soon. Once all updates are downloaded I recommend you create a complete backup. It's Best to backup a Clone of your HD to CD or external HD You will save yourself a lot of grief.
Edited by technonymous, 17 January 2014 - 08:32 PM.