Hi rkaufmann87, thanks for your reply!
Unfortunately, I had a look at the page you referred me to and there's nothing there that actually helps me.
Basically I have a 21.5" mid-2010 iMac (iMac11,2). This iMac shipped with install DVDs, but for some reason the version of AHT was incorrect. More specifically, this iMac requires AHT version 3A195, but the bundled DVDs came with AHT version 3A197 and as a result I can't use the discs to reboot and run AHT.
If I try Option - D at startup the iMac does start the process for doing it online (although it shouldn't because of its vintage) but then returns an error.
So, you see, I'm at a loss except for the fact that I did at one time run AHT on this machine (but I can't remember for the life of me how I actually did it). I would have made notes, but just haven't had the time to go searching for them (although I may find time on the weekend).
Anyway, not sure what else I could do in the meantime, short of finding AHT version 3A195 somewhere to download from etc etc.
Any suggestions/tips/tricks you can offer?