not good

When the time comes to install SP3, the right approach is to first make a disk image backup of the partition containing Windows. Anything else is risky. No doubt SP3 was designed to be un-installed should the need arise, but putting your full faith in this would be a mistake.

Also, the installation process is going to exercise the heck out of the file system, so I suggest first running a thorough Check Disk. If you have any other hard disk diagnostic utilities, it would be good to insure the hard disk is healthy before installing the service pack. And a defrag can't hurt. At the very least, make a restore point.