to perform a complete backup.
Then just ignore it for a bunch of hours.
I have an iMac with 2GHz Intel Core 2 Duo and 2GB RAM and OSX 10.5.8. Last year I bought a Western Digital My Book Mac Edition 1 TB external HD for backups. All worked well initially but later there were random occasions when Time Machine would start "preparing" to backup but then stall and keep spinning. Usually I could get it going again by stopping the backup, shutting down the external HD and/or restarting the iMac.
Now I can not get past Time Machine "preparing" to backup, even after trying the previous methods of working around the problem. It spins until I give up and stop it. I can still enter Time Machine to look at files that are no longer on the internal HD. There is still 767 GB room available on the external HD.