Windows UPDATES Problems...please help!!

Ok, this is a problem that is pretty much stumping me. I have a few pcs that i did OS installs on (all had fallen victim to the famous Anti-Virus 2008 and 2009 viruses). Now the installs went fine, but i am running into a problem with every machine when i perform windows updates. The machines simply wont install the updates. Gives a very vague message saying windows update was unable to install updates....or something like that (cant remember the exact message). This is doing this on the majority of windows XP machines and it always happens after service pack 3 has been installed. How do I fix this problem?? Why wont these updates install? is there an active-x controller i'm missing or is there some kind of hot fix for this? I just dont understand why on most xp machines, the windows updates fail, but on some, they download and install with no problem. it seems like a problem on microsoft's end.