This is what I originally did. Bought a hardware keylogger and placed it between the keyboard and the computer. Eventually the IT guy logged in with his account on my PC.
Geez - you want to be careful with that sort of thing. Finding passwords on an open share or lying around on bits of paper is one thing; using hardware based keyloggers to lift passwords is a whole different matter! You need to be careful who you annoy with this sort of thing as well; whilst your company boss may be fine with it, if the IT guys own the equipment then technically they're the ones that can prosecute you over it. If they're not the ones that own the equipment then legally you should be able to get admin permissions through legal means anyway.
Keyloggers are a bit of a last resort and intensely frowned upon in checking the security of systems. Yes, they can be useful but anyone can whack a hardware keylogger into the back of a system and get someone to log onto their machine. The only way round that would be to up the physical security, locking all the computers away under the desk where no connections can be made or broken. Quite frankly that shouldn't be the case unless you're working on an absolutely mission critical system or one readily open to the public.
As for resetting the admin password discretely - there's really no such thing. Either you try and lift the hash to try and crack it (which won't get you anywhere if it's secure enough) or you reset it and hope they don't notice.
They should notice of course. If anyone reset an admin password on a machine I was working on then I'd grab as many logs as I could to try and hunt the culprit down - before wiping and re-imaging the system before I did anything else at all.
I'm not trying to spoil the fun here, I'm a firm believer that security should be tested and insecure things should be blasted open. But putting keyloggers left right and centre in my mind is going a stage too far, and could land you in big trouble even if you don't expect it. In certain situations they are appropriate tools to use and should be guarded against, but within an organisation I think employee trust should come into the equation that much.
I'd certainly far sooner I was trusted not to whack keyloggers at the back of PCs and have them open for me to adjust and swap cables over as and when I needed than having them locked down and having to ring up an official person with a key to come every time I wanted to adjust something. If I was caught doing something like that where I work then I'd be out the door before I had a chance to explain myself, and rightly so.