Ya.
Technically OEM is for one computer only. You can't transfer it to another computer, sell it to somebody else, or use for a 'upgrade'.
If there is a dispute on what is a 'new computer' it's decided by the motherboard. For instance if you bought a new harddrive, new power supply, new cpu, new memory, new motherboard, and new cdrom for your computer over a period of three years is it still the same computer?
Technically they are still suppose to activate it even if is a new motherboard if it's a replacement for bad hardware.
In practice it's up to who your talking to on the phone. They will decide and some people will activate it even if it's a completely new computer. I don't even think that Microsoft reps you talk to realy understand everything what is going on.
Retail license is tied to YOU. You can transfer it from computer to computer. One of the things about it is that then your eligable for a 'upgrade' license.
For instance if you have a XP computer now with a OEM license and you would like to upgrade it to 'vista' you technically should have to purchase a 'retail' license for Vista. However if your current XP is a 'retail' license then you can purchase the Vista 'upgrade' license for your next computer and effectively XP retail + Vista upgrade = Vista retail.
So for a retail license for the most crippled version of Vista aviable in the U.S. will probably be around 160-200 dollars. The upgrade would probably be 60-100 bucks. Probably for the same price as low-end Vista retail you could get a high-end version of Vista upgrade.
However in practice most people here would build a new computer specificly for vista and thusly purchase a Vista OEM license for that computer. Although technically the OEM license you buy from places like newegg.com is designed for people who build computers for OTHER people (not themselves), Microsoft doesn't seem to care.
It's all realy quite insane.
My suggestion is simply try calling again and tell them that your current motherboard is a replacement for a faulty original and that you need your OEM XP activated for it. More then likely that will work fine, it's what is suppose to work from what I've read.