Doesn't sound like that big of an enterprise if your imaging solution isn't full proof and your HP contract isn't good enough to get the good stuff (I've had HP engineers agree to come in on Easter Sunday to install RAM upgrades in servers, and I have their personal cell numbers on hand any time I need them).
We're not that big of a shop and no computer on the network isn't imaged with our corporate image.
Sorry if you think this is a fight, it's not really, it's my own experience. The last time I encountered a PC with an OEM image in my work in the last 13 years was when I worked for a very small IT firm (very, very small, 12 person affair, small business support) and we sold some guy a laptop for his personal use out of our Dell contract.