Got some weird behavior from my OS today. Tried verifying permissions with Disc Utility and got the "No Valid Packages" message. Checked the Apple site and this forum and found references to BaseSystem.pkg in Library/Receipts. This package was, in fact, already in my Receipts folder. Tried replacing it with one from another system running 10.4 but the problem still remains.
OS X 10.4.5
Machine Name: Power Mac G4 (AGP graphics)
Machine Model: PowerMac3,1
CPU Type: PowerPC 60? (2.1)
Number Of CPUs: 2
CPU Speed: 1.6 GHz
L2 Cache (per CPU): 512 KB
Memory: 2 GB
OS X 10.4.5
Machine Name: Power Mac G4 (AGP graphics)
Machine Model: PowerMac3,1
CPU Type: PowerPC 60? (2.1)
Number Of CPUs: 2
CPU Speed: 1.6 GHz
L2 Cache (per CPU): 512 KB
Memory: 2 GB