Forgive me for not reading every word of this post, but I didn't see this one item mentioned ....
Memory has what I like to call "the forgotten spec." I dont' know much about it, but there is 2 and 4 clock cycle SDRAM. Most motherboards don't care and will work with either. I have no idea how to tell which one you have/are buying. however, some picky name brand systems <cough Dell, cough> will ONLY accept 4 clock cycle RAM (as printed in their specifications.) I've bought generic SDRAM and NOT had it work in Dell's because of this.
The IT guy may not be aware of this and may be attributing the fact that the PC133 doesn't work in PC100 machines to the speed ... however, I'll bet dollars to donuts that if the RAM doesn't work it's this 2 or 4 clock cycled thing that I've encountered.