Okay- so the fix for this was unusual. I was chasing my tail, thinking this issue had to do with MS Installer 4.5 not wanting to run my package. Turns out the error was a false positive, which really pointed to a bigger problem. After trying to reinstall the client, and even reimaging the box the PXE rep was supposed to go on, another machine wouldnt' install a PXE rep either. Going further, when running a Vulscan from either machine, it would error, saying it couldn't download data from the VulnerabilityData folder. The preferred server where the PXE files were coming from had blocked URLs and MIME files, preventing the package from delploying. Once this was fixed, both problems resolved, and PXE installed fine.