Computer help: System No-boot after Installing XP Service Pack3

System No-boot after Installing XP Service Pack3

Hi There. After downloading Windows XP Srvice Pack 3 a message told me to restart the computer. On restarting the screen gave a DOS screen with instructions about rebooting. I tried all the 5? choices to no avail and was almost resigned to the sickening choice of having to restore completely using F10!! However, I tried the Safe Mode again after which I managed to restore to two days before.
I don't really think this should happen! but why is it? I have had 2 similar occurence a few weeks ago when I did the F10 restore. They have all been after a Windows update. But since then I had a Blue Screen for which an engineer fitted a new hard disk. Can anyone explain this, please. In fact, now, the yellow update shield is still in the Task Bar. If I look in the log of window updates it has been recorded. I have done all checks such as virus etc.
I am just reading off the engineers report: Symptons = BSOD Frequently; Manufacture=Compaq;Model=SR1420 A643K
Thanks D.