XP sp#2 fails during cleanup -- what to do???

I have an Dimension 8300, 512Mb ram, and have XP Pro with sp#1 installed. I've tried twice so far to install sp#2, and both times it failed -- in the same spot. It takes about 3 hours for the machine to get to the point during cleanup, where I get a message about the installation failing with the updating of ntdll.dll -- I get the message about sending microsoft an error report, but nothing to suggest if I can live with the state of the machine at the time (is XP stable at that point?). The first time I tried doing the install, it was via Windows Update about three weeks ago. This second time was today, using the 266 mb file that I downloaded from Microsoft's web site. Both times, I was able to recover by going into the control panel and doing an uninstall of sp#2. Any thoughts/ideas/etc? I'm getting a bit frustrated, since MS doesn't have anything on their support web site that covers this, as far as I can tell...and my machine is old enough for them to demand money for any type of support. thanks.