I had a similar problem on two Windows Server 2003 R2 x64 installs. The above suggestion helped but it then said it couldn't find 'pms-sdk.msi', which isn't actually in the installer package. Managed to figure out that it was trying to remove the 'bonjour' component. So, abort the current install, manually just remove the bonjour component via add remove progs and start the install again from scratch. Part way through it will say it needs to install 'bonjour' in order to proceed, say yes and the install will complete fully. Both my servers updated now and running fine.
Extra note: If your still having problems with the install saying it cannot access a component of 'pms-sdk.msi', it's because when you run the install, it unpacks 6 files, one of those files is called 'pms.msi', within that msi is the file you need 'pms-sdk.msi'. If you know how you can extract it manually before hand (long winded), then when the install stops with the error point it to the manually extracted file, click ok and the install will proceed cleanly.
It's taken me most of this morning to figure this out. so to save others the same aggro I've attached a rarred copy 'pms-sdk.rar' of the file 'pms-sdk.msi', the install is looking for.
I don't which to scare monger, but please ensure you have AntiVirus of some sort installed when downloading files.
Hope this helps somebody else out